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Workqueue¶

Date:: September, 2010
Author:: Tejun Heo <tj@kernel.org>
Author:: Florian Mickler <florian@mickler.org>

Introduction¶

There are many cases where an asynchronous process execution context is needed and the workqueue (wq) API is the most commonly used mechanism for such cases.

When such an asynchronous execution context is needed, a work item describing which function to execute is put on a queue. An independent thread serves as the asynchronous execution context. The queue is called workqueue and the thread is called worker.

While there are work items on the workqueue the worker executes the functions associated with the work items one after the other. When there is no work item left on the workqueue the worker becomes idle. When a new work item gets queued, the worker begins executing again.

Why Concurrency Managed Workqueue?¶

In the original wq implementation, a multi threaded (MT) wq had one worker thread per CPU and a single threaded (ST) wq had one worker thread system-wide. A single MT wq needed to keep around the same number of workers as the number of CPUs. The kernel grew a lot of MT wq users over the years and with the number of CPU cores continuously rising, some systems saturated the default 32k PID space just booting up.

Although MT wq wasted a lot of resource, the level of concurrency provided was unsatisfactory. The limitation was common to both ST and MT wq albeit less severe on MT. Each wq maintained its own separate worker pool. An MT wq could provide only one execution context per CPU while an ST wq one for the whole system. Work items had to compete for those very limited execution contexts leading to various problems including proneness to deadlocks around the single execution context.

The tension between the provided level of concurrency and resource usage also forced its users to make unnecessary tradeoffs like libata choosing to use ST wq for polling PIOs and accepting an unnecessary limitation that no two polling PIOs can progress at the same time. As MT wq don’t provide much better concurrency, users which require higher level of concurrency, like async or fscache, had to implement their own thread pool.

Concurrency Managed Workqueue (cmwq) is a reimplementation of wq with focus on the following goals.

Maintain compatibility with the original workqueue API.
Use per-CPU unified worker pools shared by all wq to provide flexible level of concurrency on demand without wasting a lot of resource.
Automatically regulate worker pool and level of concurrency so that the API users don’t need to worry about such details.

The Design¶

In order to ease the asynchronous execution of functions a new abstraction, the work item, is introduced.

A work item is a simple struct that holds a pointer to the function that is to be executed asynchronously. Whenever a driver or subsystem wants a function to be executed asynchronously it has to set up a work item pointing to that function and queue that work item on a workqueue.

A work item can be executed in either a thread or the BH (softirq) context.

For threaded workqueues, special purpose threads, called [k]workers, execute the functions off of the queue, one after the other. If no work is queued, the worker threads become idle. These worker threads are managed in worker-pools.

The cmwq design differentiates between the user-facing workqueues that subsystems and drivers queue work items on and the backend mechanism which manages worker-pools and processes the queued work items.

There are two worker-pools, one for normal work items and the other for high priority ones, for each possible CPU and some extra worker-pools to serve work items queued on unbound workqueues - the number of these backing pools is dynamic.

BH workqueues use the same framework. However, as there can only be one concurrent execution context, there’s no need to worry about concurrency. Each per-CPU BH worker pool contains only one pseudo worker which represents the BH execution context. A BH workqueue can be considered a convenience interface to softirq.

Subsystems and drivers can create and queue work items through special workqueue API functions as they see fit. They can influence some aspects of the way the work items are executed by setting flags on the workqueue they are putting the work item on. These flags include things like CPU locality, concurrency limits, priority and more. To get a detailed overview refer to the API description of alloc_workqueue() below.

When a work item is queued to a workqueue, the target worker-pool is determined according to the queue parameters and workqueue attributes and appended on the shared worklist of the worker-pool. For example, unless specifically overridden, a work item of a bound workqueue will be queued on the worklist of either normal or highpri worker-pool that is associated to the CPU the issuer is running on.

For any thread pool implementation, managing the concurrency level (how many execution contexts are active) is an important issue. cmwq tries to keep the concurrency at a minimal but sufficient level. Minimal to save resources and sufficient in that the system is used at its full capacity.

Each worker-pool bound to an actual CPU implements concurrency management by hooking into the scheduler. The worker-pool is notified whenever an active worker wakes up or sleeps and keeps track of the number of the currently runnable workers. Generally, work items are not expected to hog a CPU and consume many cycles. That means maintaining just enough concurrency to prevent work processing from stalling should be optimal. As long as there are one or more runnable workers on the CPU, the worker-pool doesn’t start execution of a new work, but, when the last running worker goes to sleep, it immediately schedules a new worker so that the CPU doesn’t sit idle while there are pending work items. This allows using a minimal number of workers without losing execution bandwidth.

Keeping idle workers around doesn’t cost other than the memory space for kthreads, so cmwq holds onto idle ones for a while before killing them.

For unbound workqueues, the number of backing pools is dynamic. Unbound workqueue can be assigned custom attributes using apply_workqueue_attrs() and workqueue will automatically create backing worker pools matching the attributes. The responsibility of regulating concurrency level is on the users. There is also a flag to mark a bound wq to ignore the concurrency management. Please refer to the API section for details.

Forward progress guarantee relies on that workers can be created when more execution contexts are necessary, which in turn is guaranteed through the use of rescue workers. All work items which might be used on code paths that handle memory reclaim are required to be queued on wq’s that have a rescue-worker reserved for execution under memory pressure. Else it is possible that the worker-pool deadlocks waiting for execution contexts to free up.

Application Programming Interface (API)¶

alloc_workqueue() allocates a wq. The original create_*workqueue() functions are deprecated and scheduled for removal. alloc_workqueue() takes three arguments - @name, @flags and @max_active. @name is the name of the wq and also used as the name of the rescuer thread if there is one.

A wq no longer manages execution resources but serves as a domain for forward progress guarantee, flush and work item attributes. @flags and @max_active control how work items are assigned execution resources, scheduled and executed.

`flags`¶

WQ_BH

BH workqueues can be considered a convenience interface to softirq. BH workqueues are always per-CPU and all BH work items are executed in the queueing CPU’s softirq context in the queueing order.

All BH workqueues must have 0 max_active and WQ_HIGHPRI is the only allowed additional flag.

BH work items cannot sleep. All other features such as delayed queueing, flushing and canceling are supported.

WQ_UNBOUND

Work items queued to an unbound wq are served by the special worker-pools which host workers which are not bound to any specific CPU. This makes the wq behave as a simple execution context provider without concurrency management. The unbound worker-pools try to start execution of work items as soon as possible. Unbound wq sacrifices locality but is useful for the following cases.

Wide fluctuation in the concurrency level requirement is expected and using bound wq may end up creating large number of mostly unused workers across different CPUs as the issuer hops through different CPUs.
Long running CPU intensive workloads which can be better managed by the system scheduler.

WQ_FREEZABLE

A freezable wq participates in the freeze phase of the system suspend operations. Work items on the wq are drained and no new work item starts execution until thawed.

WQ_MEM_RECLAIM

All wq which might be used in the memory reclaim paths MUST have this flag set. The wq is guaranteed to have at least one execution context regardless of memory pressure.

WQ_HIGHPRI

Work items of a highpri wq are queued to the highpri worker-pool of the target cpu. Highpri worker-pools are served by worker threads with elevated nice level.

Note that normal and highpri worker-pools don’t interact with each other. Each maintains its separate pool of workers and implements concurrency management among its workers.

WQ_CPU_INTENSIVE

Work items of a CPU intensive wq do not contribute to the concurrency level. In other words, runnable CPU intensive work items will not prevent other work items in the same worker-pool from starting execution. This is useful for bound work items which are expected to hog CPU cycles so that their execution is regulated by the system scheduler.

Although CPU intensive work items don’t contribute to the concurrency level, start of their executions is still regulated by the concurrency management and runnable non-CPU-intensive work items can delay execution of CPU intensive work items.

This flag is meaningless for unbound wq.

`max_active`¶

@max_active determines the maximum number of execution contexts per CPU which can be assigned to the work items of a wq. For example, with @max_active of 16, at most 16 work items of the wq can be executing at the same time per CPU. This is always a per-CPU attribute, even for unbound workqueues.

The maximum limit for @max_active is 512 and the default value used when 0 is specified is 256. These values are chosen sufficiently high such that they are not the limiting factor while providing protection in runaway cases.

The number of active work items of a wq is usually regulated by the users of the wq, more specifically, by how many work items the users may queue at the same time. Unless there is a specific need for throttling the number of active work items, specifying ‘0’ is recommended.

Some users depend on strict execution ordering where only one work item is in flight at any given time and the work items are processed in queueing order. While the combination of @max_active of 1 and WQ_UNBOUND used to achieve this behavior, this is no longer the case. Use alloc_ordered_queue() instead.

Example Execution Scenarios¶

The following example execution scenarios try to illustrate how cmwq behave under different configurations.

Work items w0, w1, w2 are queued to a bound wq q0 on the same CPU. w0 burns CPU for 5ms then sleeps for 10ms then burns CPU for 5ms again before finishing. w1 and w2 burn CPU for 5ms then sleep for 10ms.

Ignoring all other tasks, works and processing overhead, and assuming simple FIFO scheduling, the following is one highly simplified version of possible sequences of events with the original wq.

TIME IN MSECS  EVENT
            w0 starts and burns CPU
            w0 sleeps
           w0 wakes up and burns CPU
           w0 finishes
           w1 starts and burns CPU
           w1 sleeps
           w1 wakes up and finishes
           w2 starts and burns CPU
           w2 sleeps
           w2 wakes up and finishes

And with cmwq with @max_active >= 3,

TIME IN MSECS  EVENT
            w0 starts and burns CPU
            w0 sleeps
            w1 starts and burns CPU
           w1 sleeps
           w2 starts and burns CPU
           w2 sleeps
           w0 wakes up and burns CPU
           w0 finishes
           w1 wakes up and finishes
           w2 wakes up and finishes

If @max_active == 2,

TIME IN MSECS  EVENT
            w0 starts and burns CPU
            w0 sleeps
            w1 starts and burns CPU
           w1 sleeps
           w0 wakes up and burns CPU
           w0 finishes
           w1 wakes up and finishes
           w2 starts and burns CPU
           w2 sleeps
           w2 wakes up and finishes

Now, let’s assume w1 and w2 are queued to a different wq q1 which has WQ_CPU_INTENSIVE set,

TIME IN MSECS  EVENT
            w0 starts and burns CPU
            w0 sleeps
            w1 and w2 start and burn CPU
           w1 sleeps
           w2 sleeps
           w0 wakes up and burns CPU
           w0 finishes
           w1 wakes up and finishes
           w2 wakes up and finishes

Guidelines¶

Do not forget to use WQ_MEM_RECLAIM if a wq may process work items which are used during memory reclaim. Each wq with WQ_MEM_RECLAIM set has an execution context reserved for it. If there is dependency among multiple work items used during memory reclaim, they should be queued to separate wq each with WQ_MEM_RECLAIM.
Unless strict ordering is required, there is no need to use ST wq.
Unless there is a specific need, using 0 for @max_active is recommended. In most use cases, concurrency level usually stays well under the default limit.
A wq serves as a domain for forward progress guarantee (WQ_MEM_RECLAIM, flush and work item attributes. Work items which are not involved in memory reclaim and don’t need to be flushed as a part of a group of work items, and don’t require any special attribute, can use one of the system wq. There is no difference in execution characteristics between using a dedicated wq and a system wq.
Unless work items are expected to consume a huge amount of CPU cycles, using a bound wq is usually beneficial due to the increased level of locality in wq operations and work item execution.

Affinity Scopes¶

An unbound workqueue groups CPUs according to its affinity scope to improve cache locality. For example, if a workqueue is using the default affinity scope of “cache”, it will group CPUs according to last level cache boundaries. A work item queued on the workqueue will be assigned to a worker on one of the CPUs which share the last level cache with the issuing CPU. Once started, the worker may or may not be allowed to move outside the scope depending on the affinity_strict setting of the scope.

Workqueue currently supports the following affinity scopes.

default: Use the scope in module parameter workqueue.default_affinity_scope which is always set to one of the scopes below.
cpu: CPUs are not grouped. A work item issued on one CPU is processed by a worker on the same CPU. This makes unbound workqueues behave as per-cpu workqueues without concurrency management.
smt: CPUs are grouped according to SMT boundaries. This usually means that the logical threads of each physical CPU core are grouped together.
cache: CPUs are grouped according to cache boundaries. Which specific cache boundary is used is determined by the arch code. L3 is used in a lot of cases. This is the default affinity scope.
numa: CPUs are grouped according to NUMA boundaries.
system: All CPUs are put in the same group. Workqueue makes no effort to process a work item on a CPU close to the issuing CPU.

The default affinity scope can be changed with the module parameter workqueue.default_affinity_scope and a specific workqueue’s affinity scope can be changed using apply_workqueue_attrs().

If WQ_SYSFS is set, the workqueue will have the following affinity scope related interface files under its /sys/devices/virtual/workqueue/WQ_NAME/ directory.

affinity_scope

Read to see the current affinity scope. Write to change.

When default is the current scope, reading this file will also show the current effective scope in parentheses, for example, default (cache).

affinity_strict

0 by default indicating that affinity scopes are not strict. When a work item starts execution, workqueue makes a best-effort attempt to ensure that the worker is inside its affinity scope, which is called repatriation. Once started, the scheduler is free to move the worker anywhere in the system as it sees fit. This enables benefiting from scope locality while still being able to utilize other CPUs if necessary and available.

If set to 1, all workers of the scope are guaranteed always to be in the scope. This may be useful when crossing affinity scopes has other implications, for example, in terms of power consumption or workload isolation. Strict NUMA scope can also be used to match the workqueue behavior of older kernels.

Affinity Scopes and Performance¶

It’d be ideal if an unbound workqueue’s behavior is optimal for vast majority of use cases without further tuning. Unfortunately, in the current kernel, there exists a pronounced trade-off between locality and utilization necessitating explicit configurations when workqueues are heavily used.

Higher locality leads to higher efficiency where more work is performed for the same number of consumed CPU cycles. However, higher locality may also cause lower overall system utilization if the work items are not spread enough across the affinity scopes by the issuers. The following performance testing with dm-crypt clearly illustrates this trade-off.

The tests are run on a CPU with 12-cores/24-threads split across four L3 caches (AMD Ryzen 9 3900x). CPU clock boost is turned off for consistency. /dev/dm-0 is a dm-crypt device created on NVME SSD (Samsung 990 PRO) and opened with cryptsetup with default settings.

Scenario 1: Enough issuers and work spread across the machine¶

The command used:

$ fio --filename=/dev/dm-0 --direct=1 --rw=randrw --bs=32k --ioengine=libaio \
  --iodepth=64 --runtime=60 --numjobs=24 --time_based --group_reporting \
  --name=iops-test-job --verify=sha512

There are 24 issuers, each issuing 64 IOs concurrently. --verify=sha512 makes fio generate and read back the content each time which makes execution locality matter between the issuer and kcryptd. The following are the read bandwidths and CPU utilizations depending on different affinity scope settings on kcryptd measured over five runs. Bandwidths are in MiBps, and CPU util in percents.

Affinity	Bandwidth (MiBps)	CPU util (%)
system	1159.40 ±1.34	99.31 ±0.02
cache	1166.40 ±0.89	99.34 ±0.01
cache (strict)	1166.00 ±0.71	99.35 ±0.01

With enough issuers spread across the system, there is no downside to “cache”, strict or otherwise. All three configurations saturate the whole machine but the cache-affine ones outperform by 0.6% thanks to improved locality.

Scenario 2: Fewer issuers, enough work for saturation¶

The command used:

$ fio --filename=/dev/dm-0 --direct=1 --rw=randrw --bs=32k \
  --ioengine=libaio --iodepth=64 --runtime=60 --numjobs=8 \
  --time_based --group_reporting --name=iops-test-job --verify=sha512

The only difference from the previous scenario is --numjobs=8. There are a third of the issuers but is still enough total work to saturate the system.

Affinity	Bandwidth (MiBps)	CPU util (%)
system	1155.40 ±0.89	97.41 ±0.05
cache	1154.40 ±1.14	96.15 ±0.09
cache (strict)	1112.00 ±4.64	93.26 ±0.35

This is more than enough work to saturate the system. Both “system” and “cache” are nearly saturating the machine but not fully. “cache” is using less CPU but the better efficiency puts it at the same bandwidth as “system”.

Eight issuers moving around over four L3 cache scope still allow “cache (strict)” to mostly saturate the machine but the loss of work conservation is now starting to hurt with 3.7% bandwidth loss.

Scenario 3: Even fewer issuers, not enough work to saturate¶

The command used:

$ fio --filename=/dev/dm-0 --direct=1 --rw=randrw --bs=32k \
  --ioengine=libaio --iodepth=64 --runtime=60 --numjobs=4 \
  --time_based --group_reporting --name=iops-test-job --verify=sha512

Again, the only difference is --numjobs=4. With the number of issuers reduced to four, there now isn’t enough work to saturate the whole system and the bandwidth becomes dependent on completion latencies.

Affinity	Bandwidth (MiBps)	CPU util (%)
system	993.60 ±1.82	75.49 ±0.06
cache	973.40 ±1.52	74.90 ±0.07
cache (strict)	828.20 ±4.49	66.84 ±0.29

Now, the tradeoff between locality and utilization is clearer. “cache” shows 2% bandwidth loss compared to “system” and “cache (struct)” whopping 20%.

Conclusion and Recommendations¶

In the above experiments, the efficiency advantage of the “cache” affinity scope over “system” is, while consistent and noticeable, small. However, the impact is dependent on the distances between the scopes and may be more pronounced in processors with more complex topologies.

While the loss of work-conservation in certain scenarios hurts, it is a lot better than “cache (strict)” and maximizing workqueue utilization is unlikely to be the common case anyway. As such, “cache” is the default affinity scope for unbound pools.

As there is no one option which is great for most cases, workqueue usages that may consume a significant amount of CPU are recommended to configure the workqueues using apply_workqueue_attrs() and/or enable WQ_SYSFS.
An unbound workqueue with strict “cpu” affinity scope behaves the same as WQ_CPU_INTENSIVE per-cpu workqueue. There is no real advanage to the latter and an unbound workqueue provides a lot more flexibility.
Affinity scopes are introduced in Linux v6.5. To emulate the previous behavior, use strict “numa” affinity scope.
The loss of work-conservation in non-strict affinity scopes is likely originating from the scheduler. There is no theoretical reason why the kernel wouldn’t be able to do the right thing and maintain work-conservation in most cases. As such, it is possible that future scheduler improvements may make most of these tunables unnecessary.

Examining Configuration¶

Use tools/workqueue/wq_dump.py to examine unbound CPU affinity configuration, worker pools and how workqueues map to the pools:

$ tools/workqueue/wq_dump.py
Affinity Scopes
===============
wq_unbound_cpumask=0000000f

CPU
  nr_pods  4
  pod_cpus [0]=00000001 [1]=00000002 [2]=00000004 [3]=00000008
  pod_node [0]=0 [1]=0 [2]=1 [3]=1
  cpu_pod  [0]=0 [1]=1 [2]=2 [3]=3

SMT
  nr_pods  4
  pod_cpus [0]=00000001 [1]=00000002 [2]=00000004 [3]=00000008
  pod_node [0]=0 [1]=0 [2]=1 [3]=1
  cpu_pod  [0]=0 [1]=1 [2]=2 [3]=3

CACHE (default)
  nr_pods  2
  pod_cpus [0]=00000003 [1]=0000000c
  pod_node [0]=0 [1]=1
  cpu_pod  [0]=0 [1]=0 [2]=1 [3]=1

NUMA
  nr_pods  2
  pod_cpus [0]=00000003 [1]=0000000c
  pod_node [0]=0 [1]=1
  cpu_pod  [0]=0 [1]=0 [2]=1 [3]=1

SYSTEM
  nr_pods  1
  pod_cpus [0]=0000000f
  pod_node [0]=-1
  cpu_pod  [0]=0 [1]=0 [2]=0 [3]=0

Worker Pools
============
pool[00] ref= 1 nice=  0 idle/workers=  4/  4 cpu=  0
pool[01] ref= 1 nice=-20 idle/workers=  2/  2 cpu=  0
pool[02] ref= 1 nice=  0 idle/workers=  4/  4 cpu=  1
pool[03] ref= 1 nice=-20 idle/workers=  2/  2 cpu=  1
pool[04] ref= 1 nice=  0 idle/workers=  4/  4 cpu=  2
pool[05] ref= 1 nice=-20 idle/workers=  2/  2 cpu=  2
pool[06] ref= 1 nice=  0 idle/workers=  3/  3 cpu=  3
pool[07] ref= 1 nice=-20 idle/workers=  2/  2 cpu=  3
pool[08] ref=42 nice=  0 idle/workers=  6/  6 cpus=0000000f
pool[09] ref=28 nice=  0 idle/workers=  3/  3 cpus=00000003
pool[10] ref=28 nice=  0 idle/workers= 17/ 17 cpus=0000000c
pool[11] ref= 1 nice=-20 idle/workers=  1/  1 cpus=0000000f
pool[12] ref= 2 nice=-20 idle/workers=  1/  1 cpus=00000003
pool[13] ref= 2 nice=-20 idle/workers=  1/  1 cpus=0000000c

Workqueue CPU -> pool
=====================
[    workqueue \ CPU              0  1  2  3 dfl]
events                   percpu   0  2  4  6
events_highpri           percpu   1  3  5  7
events_long              percpu   0  2  4  6
events_unbound           unbound  9  9 10 10  8
events_freezable         percpu   0  2  4  6
events_power_efficient   percpu   0  2  4  6
events_freezable_pwr_ef  percpu   0  2  4  6
rcu_gp                   percpu   0  2  4  6
rcu_par_gp               percpu   0  2  4  6
slub_flushwq             percpu   0  2  4  6
netns                    ordered  8  8  8  8  8
...

See the command’s help message for more info.

Monitoring¶

Use tools/workqueue/wq_monitor.py to monitor workqueue operations:

$ tools/workqueue/wq_monitor.py events
                            total  infl  CPUtime  CPUhog CMW/RPR  mayday rescued
events                      18545     0      6.1       0       5       -       -
events_highpri                  8     0      0.0       0       0       -       -
events_long                     3     0      0.0       0       0       -       -
events_unbound              38306     0      0.1       -       7       -       -
events_freezable                0     0      0.0       0       0       -       -
events_power_efficient      29598     0      0.2       0       0       -       -
events_freezable_pwr_ef        10     0      0.0       0       0       -       -
sock_diag_events                0     0      0.0       0       0       -       -

                            total  infl  CPUtime  CPUhog CMW/RPR  mayday rescued
events                      18548     0      6.1       0       5       -       -
events_highpri                  8     0      0.0       0       0       -       -
events_long                     3     0      0.0       0       0       -       -
events_unbound              38322     0      0.1       -       7       -       -
events_freezable                0     0      0.0       0       0       -       -
events_power_efficient      29603     0      0.2       0       0       -       -
events_freezable_pwr_ef        10     0      0.0       0       0       -       -
sock_diag_events                0     0      0.0       0       0       -       -

...

See the command’s help message for more info.

Debugging¶

Because the work functions are executed by generic worker threads there are a few tricks needed to shed some light on misbehaving workqueue users.

Worker threads show up in the process list as:

root      5671  0.0  0.0      0     0 ?        S    12:07   0:00 [kworker/0:1]
root      5672  0.0  0.0      0     0 ?        S    12:07   0:00 [kworker/1:2]
root      5673  0.0  0.0      0     0 ?        S    12:12   0:00 [kworker/0:0]
root      5674  0.0  0.0      0     0 ?        S    12:13   0:00 [kworker/1:0]

If kworkers are going crazy (using too much cpu), there are two types of possible problems:

Something being scheduled in rapid succession

A single work item that consumes lots of cpu cycles

The first one can be tracked using tracing:

$ echo workqueue:workqueue_queue_work > /sys/kernel/tracing/set_event
$ cat /sys/kernel/tracing/trace_pipe > out.txt
(wait a few secs)
^C

If something is busy looping on work queueing, it would be dominating the output and the offender can be determined with the work item function.

For the second type of problems it should be possible to just check the stack trace of the offending worker thread.

$ cat /proc/THE_OFFENDING_KWORKER/stack

The work item’s function should be trivially visible in the stack trace.

Non-reentrance Conditions¶

Workqueue guarantees that a work item cannot be re-entrant if the following conditions hold after a work item gets queued:

The work function hasn’t been changed.

No one queues the work item to another workqueue.

The work item hasn’t been reinitiated.

In other words, if the above conditions hold, the work item is guaranteed to be executed by at most one worker system-wide at any given time.

Note that requeuing the work item (to the same queue) in the self function doesn’t break these conditions, so it’s safe to do. Otherwise, caution is required when breaking the conditions inside a work function.

Kernel Inline Documentations Reference¶

struct workqueue_attrs¶: A struct for workqueue attributes.

Definition:

struct workqueue_attrs {
    int nice;
    cpumask_var_t cpumask;
    cpumask_var_t __pod_cpumask;
    bool affn_strict;
    enum wq_affn_scope affn_scope;
    bool ordered;
};

Members

nice

nice level

cpumask

allowed CPUs

Work items in this workqueue are affine to these CPUs and not allowed to execute on other CPUs. A pool serving a workqueue must have the same cpumask.

__pod_cpumask

internal attribute used to create per-pod pools

Internal use only.

Per-pod unbound worker pools are used to improve locality. Always a subset of ->cpumask. A workqueue can be associated with multiple worker pools with disjoint __pod_cpumask’s. Whether the enforcement of a pool’s __pod_cpumask is strict depends on affn_strict.

affn_strict

affinity scope is strict

If clear, workqueue will make a best-effort attempt at starting the worker inside __pod_cpumask but the scheduler is free to migrate it outside.

If set, workers are only allowed to run inside __pod_cpumask.

affn_scope

unbound CPU affinity scope

CPU pods are used to improve execution locality of unbound work items. There are multiple pod types, one for each wq_affn_scope, and every CPU in the system belongs to one pod in every pod type. CPUs that belong to the same pod share the worker pool. For example, selecting WQ_AFFN_NUMA makes the workqueue use a separate worker pool for each NUMA node.

ordered

work items must be executed one by one in queueing order

Description

This can be used to change attributes of an unbound workqueue.

work_pending¶

work_pending (work)

Find out whether a work item is currently pending

Parameters

work: The work item in question

delayed_work_pending¶

delayed_work_pending (w)

Find out whether a delayable work item is currently pending

Parameters

w: The work item in question

struct workqueue_struct *alloc_workqueue(const char *fmt, unsigned int flags, int max_active, ...)¶: allocate a workqueue

Parameters

const char *fmt: printf format for the name of the workqueue
unsigned int flags: WQ_* flags
int max_active: max in-flight work items, 0 for default
...: args for fmt

Description

For a per-cpu workqueue, max_active limits the number of in-flight work items for each CPU. e.g. max_active of 1 indicates that each CPU can be executing at most one work item for the workqueue.

For unbound workqueues, max_active limits the number of in-flight work items for the whole system. e.g. max_active of 16 indicates that that there can be at most 16 work items executing for the workqueue in the whole system.

As sharing the same active counter for an unbound workqueue across multiple NUMA nodes can be expensive, max_active is distributed to each NUMA node according to the proportion of the number of online CPUs and enforced independently.

Depending on online CPU distribution, a node may end up with per-node max_active which is significantly lower than max_active, which can lead to deadlocks if the per-node concurrency limit is lower than the maximum number of interdependent work items for the workqueue.

To guarantee forward progress regardless of online CPU distribution, the concurrency limit on every node is guaranteed to be equal to or greater than min_active which is set to min(max_active, WQ_DFL_MIN_ACTIVE). This means that the sum of per-node max_active’s may be larger than max_active.

For detailed information on WQ_* flags, please refer to Workqueue.

Return

Pointer to the allocated workqueue on success, NULL on failure.

alloc_ordered_workqueue¶

alloc_ordered_workqueue (fmt, flags, args...)

allocate an ordered workqueue

Parameters

fmt: printf format for the name of the workqueue
flags: WQ_* flags (only WQ_FREEZABLE and WQ_MEM_RECLAIM are meaningful)
args...: args for fmt

Description

Allocate an ordered workqueue. An ordered workqueue executes at most one work item at any given time in the queued order. They are implemented as unbound workqueues with max_active of one.

Return

Pointer to the allocated workqueue on success, NULL on failure.

bool queue_work(struct workqueue_struct *wq, struct work_struct *work)¶: queue work on a workqueue

Parameters

struct workqueue_struct *wq: workqueue to use
struct work_struct *work: work to queue

Description

Returns false if work was already on a queue, true otherwise.

We queue the work to the CPU on which it was submitted, but if the CPU dies it can be processed by another CPU.

Memory-ordering properties: If it returns true, guarantees that all stores preceding the call to queue_work() in the program order will be visible from the CPU which will execute work by the time such work executes, e.g.,

{ x is initially 0 }

CPU0 CPU1

WRITE_ONCE(x, 1); [ work is being executed ] r0 = queue_work(wq, work); r1 = READ_ONCE(x);

Forbids: r0 == true && r1 == 0

bool queue_delayed_work(struct workqueue_struct *wq, struct delayed_work *dwork, unsigned long delay)¶: queue work on a workqueue after delay

Parameters

struct workqueue_struct *wq: workqueue to use
struct delayed_work *dwork: delayable work to queue
unsigned long delay: number of jiffies to wait before queueing

Description

Equivalent to queue_delayed_work_on() but tries to use the local CPU.

bool mod_delayed_work(struct workqueue_struct *wq, struct delayed_work *dwork, unsigned long delay)¶: modify delay of or queue a delayed work

Parameters

struct workqueue_struct *wq: workqueue to use
struct delayed_work *dwork: work to queue
unsigned long delay: number of jiffies to wait before queueing

Description

mod_delayed_work_on() on local CPU.

bool schedule_work_on(int cpu, struct work_struct *work)¶: put work task on a specific cpu

Parameters

int cpu: cpu to put the work task on
struct work_struct *work: job to be done

Description

This puts a job on a specific cpu

bool schedule_work(struct work_struct *work)¶: put work task in global workqueue

Parameters

struct work_struct *work: job to be done

Description

Returns false if work was already on the kernel-global workqueue and true otherwise.

This puts a job in the kernel-global workqueue if it was not already queued and leaves it in the same position on the kernel-global workqueue otherwise.

Shares the same memory-ordering properties of queue_work(), cf. the DocBook header of queue_work().

bool enable_and_queue_work(struct workqueue_struct *wq, struct work_struct *work)¶: Enable and queue a work item on a specific workqueue

Parameters

struct workqueue_struct *wq: The target workqueue
struct work_struct *work: The work item to be enabled and queued

Description

This function combines the operations of enable_work() and queue_work(), providing a convenient way to enable and queue a work item in a single call. It invokes enable_work() on work and then queues it if the disable depth reached 0. Returns true if the disable depth reached 0 and work is queued, and false otherwise.

Note that work is always queued when disable depth reaches zero. If the desired behavior is queueing only if certain events took place while work is disabled, the user should implement the necessary state tracking and perform explicit conditional queueing after enable_work().

bool schedule_delayed_work_on(int cpu, struct delayed_work *dwork, unsigned long delay)¶: queue work in global workqueue on CPU after delay

Parameters

int cpu: cpu to use
struct delayed_work *dwork: job to be done
unsigned long delay: number of jiffies to wait

Description

After waiting for a given time this puts a job in the kernel-global workqueue on the specified CPU.

bool schedule_delayed_work(struct delayed_work *dwork, unsigned long delay)¶: put work task in global workqueue after delay

Parameters

struct delayed_work *dwork: job to be done
unsigned long delay: number of jiffies to wait or 0 for immediate execution

Description

After waiting for a given time this puts a job in the kernel-global workqueue.

for_each_pool¶

for_each_pool (pool, pi)

iterate through all worker_pools in the system

Parameters

pool: iteration cursor
pi: integer used for iteration

Description

This must be called either with wq_pool_mutex held or RCU read locked. If the pool needs to be used beyond the locking in effect, the caller is responsible for guaranteeing that the pool stays online.

The if/else clause exists only for the lockdep assertion and can be ignored.

for_each_pool_worker¶

for_each_pool_worker (worker, pool)

iterate through all workers of a worker_pool

Parameters

worker: iteration cursor
pool: worker_pool to iterate workers of

Description

This must be called with wq_pool_attach_mutex.

The if/else clause exists only for the lockdep assertion and can be ignored.

for_each_pwq¶

for_each_pwq (pwq, wq)

iterate through all pool_workqueues of the specified workqueue

Parameters

pwq: iteration cursor
wq: the target workqueue

Description

This must be called either with wq->mutex held or RCU read locked. If the pwq needs to be used beyond the locking in effect, the caller is responsible for guaranteeing that the pwq stays online.

The if/else clause exists only for the lockdep assertion and can be ignored.

int worker_pool_assign_id(struct worker_pool *pool)¶: allocate ID and assign it to pool

Parameters

struct worker_pool *pool: the pool pointer of interest

Description

Returns 0 if ID in [0, WORK_OFFQ_POOL_NONE) is allocated and assigned successfully, -errno on failure.

struct cpumask *unbound_effective_cpumask(struct workqueue_struct *wq)¶: effective cpumask of an unbound workqueue

Parameters

struct workqueue_struct *wq: workqueue of interest

Description

wq->unbound_attrs->cpumask contains the cpumask requested by the user which is masked with wq_unbound_cpumask to determine the effective cpumask. The default pwq is always mapped to the pool with the current effective cpumask.

struct worker_pool *get_work_pool(struct work_struct *work)¶: return the worker_pool a given work was associated with

Parameters

struct work_struct *work: the work item of interest

Description

Pools are created and destroyed under wq_pool_mutex, and allows read access under RCU read lock. As such, this function should be called under wq_pool_mutex or inside of a rcu_read_lock() region.

All fields of the returned pool are accessible as long as the above mentioned locking is in effect. If the returned pool needs to be used beyond the critical section, the caller is responsible for ensuring the returned pool is and stays online.

Return

The worker_pool work was last associated with. NULL if none.

void worker_set_flags(struct worker *worker, unsigned int flags)¶: set worker flags and adjust nr_running accordingly

Parameters

struct worker *worker: self
unsigned int flags: flags to set

Description

Set flags in worker->flags and adjust nr_running accordingly.

void worker_clr_flags(struct worker *worker, unsigned int flags)¶: clear worker flags and adjust nr_running accordingly

Parameters

struct worker *worker: self
unsigned int flags: flags to clear

Description

Clear flags in worker->flags and adjust nr_running accordingly.

void worker_enter_idle(struct worker *worker)¶: enter idle state

Parameters

struct worker *worker: worker which is entering idle state

Description

worker is entering idle state. Update stats and idle timer if necessary.

LOCKING: raw_spin_lock_irq(pool->lock).

void worker_leave_idle(struct worker *worker)¶: leave idle state

Parameters

struct worker *worker: worker which is leaving idle state

Description

worker is leaving idle state. Update stats.

LOCKING: raw_spin_lock_irq(pool->lock).

struct worker *find_worker_executing_work(struct worker_pool *pool, struct work_struct *work)¶: find worker which is executing a work

Parameters

struct worker_pool *pool: pool of interest
struct work_struct *work: work to find worker for

Description

Find a worker which is executing work on pool by searching pool->busy_hash which is keyed by the address of work. For a worker to match, its current execution should match the address of work and its work function. This is to avoid unwanted dependency between unrelated work executions through a work item being recycled while still being executed.

This is a bit tricky. A work item may be freed once its execution starts and nothing prevents the freed area from being recycled for another work item. If the same work item address ends up being reused before the original execution finishes, workqueue will identify the recycled work item as currently executing and make it wait until the current execution finishes, introducing an unwanted dependency.

This function checks the work item address and work function to avoid false positives. Note that this isn’t complete as one may construct a work function which can introduce dependency onto itself through a recycled work item. Well, if somebody wants to shoot oneself in the foot that badly, there’s only so much we can do, and if such deadlock actually occurs, it should be easy to locate the culprit work function.

Context

raw_spin_lock_irq(pool->lock).

Return

Pointer to worker which is executing work if found, NULL otherwise.

void move_linked_works(struct work_struct *work, struct list_head *head, struct work_struct **nextp)¶: move linked works to a list

Parameters

struct work_struct *work: start of series of works to be scheduled
struct list_head *head: target list to append work to
struct work_struct **nextp: out parameter for nested worklist walking

Description

Schedule linked works starting from work to head. Work series to be scheduled starts at work and includes any consecutive work with WORK_STRUCT_LINKED set in its predecessor. See assign_work() for details on nextp.

Context

raw_spin_lock_irq(pool->lock).

bool assign_work(struct work_struct *work, struct worker *worker, struct work_struct **nextp)¶: assign a work item and its linked work items to a worker

Parameters

struct work_struct *work: work to assign
struct worker *worker: worker to assign to
struct work_struct **nextp: out parameter for nested worklist walking

Description

Assign work and its linked work items to worker. If work is already being executed by another worker in the same pool, it’ll be punted there.

If nextp is not NULL, it’s updated to point to the next work of the last scheduled work. This allows assign_work() to be nested inside list_for_each_entry_safe().

Returns true if work was successfully assigned to worker. false if work was punted to another worker already executing it.

bool kick_pool(struct worker_pool *pool)¶: wake up an idle worker if necessary

Parameters

struct worker_pool *pool: pool to kick

Description

pool may have pending work items. Wake up worker if necessary. Returns whether a worker was woken up.

void wq_worker_running(struct task_struct *task)¶: a worker is running again

Parameters

struct task_struct *task: task waking up

Description

This function is called when a worker returns from schedule()

void wq_worker_sleeping(struct task_struct *task)¶: a worker is going to sleep

Parameters

struct task_struct *task: task going to sleep

Description

This function is called from schedule() when a busy worker is going to sleep.

void wq_worker_tick(struct task_struct *task)¶: a scheduler tick occurred while a kworker is running

Parameters

struct task_struct *task: task currently running

Description

Called from sched_tick(). We’re in the IRQ context and the current worker’s fields which follow the ‘K’ locking rule can be accessed safely.

work_func_t wq_worker_last_func(struct task_struct *task)¶: retrieve worker’s last work function

Parameters

struct task_struct *task: Task to retrieve last work function of.

Description

Determine the last function a worker executed. This is called from the scheduler to get a worker’s last known identity.

This function is called during schedule() when a kworker is going to sleep. It’s used by psi to identify aggregation workers during dequeuing, to allow periodic aggregation to shut-off when that worker is the last task in the system or cgroup to go to sleep.

As this function doesn’t involve any workqueue-related locking, it only returns stable values when called from inside the scheduler’s queuing and dequeuing paths, when task, which must be a kworker, is guaranteed to not be processing any works.

Context

raw_spin_lock_irq(rq->lock)

Return

The last work function current executed as a worker, NULL if it hasn’t executed any work yet.

struct wq_node_nr_active *wq_node_nr_active(struct workqueue_struct *wq, int node)¶: Determine wq_node_nr_active to use

Parameters

struct workqueue_struct *wq: workqueue of interest
int node: NUMA node, can be NUMA_NO_NODE

Description

Determine wq_node_nr_active to use for wq on node. Returns:

NULL for per-cpu workqueues as they don’t need to use shared nr_active.
node_nr_active[nr_node_ids] if node is NUMA_NO_NODE.
Otherwise, node_nr_active[node].

void wq_update_node_max_active(struct workqueue_struct *wq, int off_cpu)¶: Update per-node max_actives to use

Parameters

struct workqueue_struct *wq: workqueue to update
int off_cpu: CPU that’s going down, -1 if a CPU is not going down

Description

Update wq->node_nr_active**[]->max. **wq must be unbound. max_active is distributed among nodes according to the proportions of numbers of online cpus. The result is always between wq->min_active and max_active.

void get_pwq(struct pool_workqueue *pwq)¶: get an extra reference on the specified pool_workqueue

Parameters

struct pool_workqueue *pwq: pool_workqueue to get

Description

Obtain an extra reference on pwq. The caller should guarantee that pwq has positive refcnt and be holding the matching pool->lock.

void put_pwq(struct pool_workqueue *pwq)¶: put a pool_workqueue reference

Parameters

struct pool_workqueue *pwq: pool_workqueue to put

Description

Drop a reference of pwq. If its refcnt reaches zero, schedule its destruction. The caller should be holding the matching pool->lock.

void put_pwq_unlocked(struct pool_workqueue *pwq)¶: put_pwq() with surrounding pool lock/unlock

Parameters

struct pool_workqueue *pwq: pool_workqueue to put (can be NULL)

Description

put_pwq() with locking. This function also allows NULL pwq.

bool pwq_activate_work(struct pool_workqueue *pwq, struct work_struct *work)¶: Activate a work item if inactive

Parameters

struct pool_workqueue *pwq: pool_workqueue work belongs to
struct work_struct *work: work item to activate

Description

Returns true if activated. false if already active.

bool pwq_tryinc_nr_active(struct pool_workqueue *pwq, bool fill)¶: Try to increment nr_active for a pwq

Parameters

struct pool_workqueue *pwq: pool_workqueue of interest
bool fill: max_active may have increased, try to increase concurrency level

Description

Try to increment nr_active for pwq. Returns true if an nr_active count is successfully obtained. false otherwise.

bool pwq_activate_first_inactive(struct pool_workqueue *pwq, bool fill)¶: Activate the first inactive work item on a pwq

Parameters

struct pool_workqueue *pwq: pool_workqueue of interest
bool fill: max_active may have increased, try to increase concurrency level

Description

Activate the first inactive work item of pwq if available and allowed by max_active limit.

Returns true if an inactive work item has been activated. false if no inactive work item is found or max_active limit is reached.

void unplug_oldest_pwq(struct workqueue_struct *wq)¶: unplug the oldest pool_workqueue

Parameters

struct workqueue_struct *wq: workqueue_struct where its oldest pwq is to be unplugged

Description

This function should only be called for ordered workqueues where only the oldest pwq is unplugged, the others are plugged to suspend execution to ensure proper work item ordering:

dfl_pwq --------------+     [P] - plugged
                      |
                      v
pwqs -> A -> B [P] -> C [P] (newest)
        |    |        |
        1    3        5
        |    |        |
        2    4        6

When the oldest pwq is drained and removed, this function should be called to unplug the next oldest one to start its work item execution. Note that pwq’s are linked into wq->pwqs with the oldest first, so the first one in the list is the oldest.

void node_activate_pending_pwq(struct wq_node_nr_active *nna, struct worker_pool *caller_pool)¶: Activate a pending pwq on a wq_node_nr_active

Parameters

struct wq_node_nr_active *nna: wq_node_nr_active to activate a pending pwq for
struct worker_pool *caller_pool: worker_pool the caller is locking

Description

Activate a pwq in nna->pending_pwqs. Called with caller_pool locked. caller_pool may be unlocked and relocked to lock other worker_pools.

void pwq_dec_nr_active(struct pool_workqueue *pwq)¶: Retire an active count

Parameters

struct pool_workqueue *pwq: pool_workqueue of interest

Description

Decrement pwq’s nr_active and try to activate the first inactive work item. For unbound workqueues, this function may temporarily drop pwq->pool->lock.

void pwq_dec_nr_in_flight(struct pool_workqueue *pwq, unsigned long work_data)¶: decrement pwq’s nr_in_flight

Parameters

struct pool_workqueue *pwq: pwq of interest
unsigned long work_data: work_data of work which left the queue

Description

A work either has completed or is removed from pending queue, decrement nr_in_flight of its pwq and handle workqueue flushing.

NOTE

For unbound workqueues, this function may temporarily drop pwq->pool->lock and thus should be called after all other state updates for the in-flight work item is complete.

Context

raw_spin_lock_irq(pool->lock).

int try_to_grab_pending(struct work_struct *work, u32 cflags, unsigned long *irq_flags)¶: steal work item from worklist and disable irq

Parameters

struct work_struct *work: work item to steal
u32 cflags: WORK_CANCEL_ flags
unsigned long *irq_flags: place to store irq state

Description

Try to grab PENDING bit of work. This function can handle work in any stable state - idle, on timer or on worklist.

On successful return, >= 0, irq is disabled and the caller is responsible for releasing it using local_irq_restore(*irq_flags).

This function is safe to call from any context including IRQ handler.

Return

1

if work was pending and we successfully stole PENDING

0

if work was idle and we claimed PENDING

-EAGAIN

if PENDING couldn’t be grabbed at the moment, safe to busy-retry

Note

On >= 0 return, the caller owns work’s PENDING bit. To avoid getting interrupted while holding PENDING and work off queue, irq must be disabled on entry. This, combined with delayed_work->timer being irqsafe, ensures that we return -EAGAIN for finite short period of time.

bool work_grab_pending(struct work_struct *work, u32 cflags, unsigned long *irq_flags)¶: steal work item from worklist and disable irq

Parameters

struct work_struct *work: work item to steal
u32 cflags: WORK_CANCEL_ flags
unsigned long *irq_flags: place to store IRQ state

Description

Grab PENDING bit of work. work can be in any stable state - idle, on timer or on worklist.

Can be called from any context. IRQ is disabled on return with IRQ state stored in *irq_flags. The caller is responsible for re-enabling it using local_irq_restore().

Returns true if work was pending. false if idle.

void insert_work(struct pool_workqueue *pwq, struct work_struct *work, struct list_head *head, unsigned int extra_flags)¶: insert a work into a pool

Parameters

struct pool_workqueue *pwq: pwq work belongs to
struct work_struct *work: work to insert
struct list_head *head: insertion point
unsigned int extra_flags: extra WORK_STRUCT_* flags to set

Description

Insert work which belongs to pwq after head. extra_flags is or’d to work_struct flags.

Context

raw_spin_lock_irq(pool->lock).

bool queue_work_on(int cpu, struct workqueue_struct *wq, struct work_struct *work)¶: queue work on specific cpu

Parameters

int cpu: CPU number to execute work on
struct workqueue_struct *wq: workqueue to use
struct work_struct *work: work to queue

Description

We queue the work to a specific CPU, the caller must ensure it can’t go away. Callers that fail to ensure that the specified CPU cannot go away will execute on a randomly chosen CPU. But note well that callers specifying a CPU that never has been online will get a splat.

Return

false if work was already on a queue, true otherwise.

int select_numa_node_cpu(int node)¶: Select a CPU based on NUMA node

Parameters

int node: NUMA node ID that we want to select a CPU from

Description

This function will attempt to find a “random” cpu available on a given node. If there are no CPUs available on the given node it will return WORK_CPU_UNBOUND indicating that we should just schedule to any available CPU if we need to schedule this work.

bool queue_work_node(int node, struct workqueue_struct *wq, struct work_struct *work)¶: queue work on a “random” cpu for a given NUMA node

Parameters

int node: NUMA node that we are targeting the work for
struct workqueue_struct *wq: workqueue to use
struct work_struct *work: work to queue

Description

We queue the work to a “random” CPU within a given NUMA node. The basic idea here is to provide a way to somehow associate work with a given NUMA node.

This function will only make a best effort attempt at getting this onto the right NUMA node. If no node is requested or the requested node is offline then we just fall back to standard queue_work behavior.

Currently the “random” CPU ends up being the first available CPU in the intersection of cpu_online_mask and the cpumask of the node, unless we are running on the node. In that case we just use the current CPU.

Return

false if work was already on a queue, true otherwise.

bool queue_delayed_work_on(int cpu, struct workqueue_struct *wq, struct delayed_work *dwork, unsigned long delay)¶: queue work on specific CPU after delay

Parameters

int cpu: CPU number to execute work on
struct workqueue_struct *wq: workqueue to use
struct delayed_work *dwork: work to queue
unsigned long delay: number of jiffies to wait before queueing

Return

false if work was already on a queue, true otherwise. If delay is zero and dwork is idle, it will be scheduled for immediate execution.

bool mod_delayed_work_on(int cpu, struct workqueue_struct *wq, struct delayed_work *dwork, unsigned long delay)¶: modify delay of or queue a delayed work on specific CPU

Parameters

int cpu: CPU number to execute work on
struct workqueue_struct *wq: workqueue to use
struct delayed_work *dwork: work to queue
unsigned long delay: number of jiffies to wait before queueing

Description

If dwork is idle, equivalent to queue_delayed_work_on(); otherwise, modify dwork’s timer so that it expires after delay. If delay is zero, work is guaranteed to be scheduled immediately regardless of its current state.

This function is safe to call from any context including IRQ handler. See try_to_grab_pending() for details.

Return

false if dwork was idle and queued, true if dwork was pending and its timer was modified.

bool queue_rcu_work(struct workqueue_struct *wq, struct rcu_work *rwork)¶: queue work after a RCU grace period

Parameters

struct workqueue_struct *wq: workqueue to use
struct rcu_work *rwork: work to queue

Return

false if rwork was already pending, true otherwise. Note that a full RCU grace period is guaranteed only after a true return. While rwork is guaranteed to be executed after a false return, the execution may happen before a full RCU grace period has passed.

void worker_attach_to_pool(struct worker *worker, struct worker_pool *pool)¶: attach a worker to a pool

Parameters

struct worker *worker: worker to be attached
struct worker_pool *pool: the target pool

Description

Attach worker to pool. Once attached, the WORKER_UNBOUND flag and cpu-binding of worker are kept coordinated with the pool across cpu-[un]hotplugs.

void worker_detach_from_pool(struct worker *worker)¶: detach a worker from its pool

Parameters

struct worker *worker: worker which is attached to its pool

Description

Undo the attaching which had been done in worker_attach_to_pool(). The caller worker shouldn’t access to the pool after detached except it has other reference to the pool.

struct worker *create_worker(struct worker_pool *pool)¶: create a new workqueue worker

Parameters

struct worker_pool *pool: pool the new worker will belong to

Description

Create and start a new worker which is attached to pool.

Context

Might sleep. Does GFP_KERNEL allocations.

Return

Pointer to the newly created worker.

void set_worker_dying(struct worker *worker, struct list_head *list)¶: Tag a worker for destruction

Parameters

struct worker *worker: worker to be destroyed
struct list_head *list: transfer worker away from its pool->idle_list and into list

Description

Tag worker for destruction and adjust pool stats accordingly. The worker should be idle.

Context

raw_spin_lock_irq(pool->lock).

void idle_worker_timeout(struct timer_list *t)¶: check if some idle workers can now be deleted.

Parameters

struct timer_list *t: The pool’s idle_timer that just expired

Description

The timer is armed in worker_enter_idle(). Note that it isn’t disarmed in worker_leave_idle(), as a worker flicking between idle and active while its pool is at the too_many_workers() tipping point would cause too much timer housekeeping overhead. Since IDLE_WORKER_TIMEOUT is long enough, we just let it expire and re-evaluate things from there.

void idle_cull_fn(struct work_struct *work)¶: cull workers that have been idle for too long.

Parameters

struct work_struct *work: the pool’s work for handling these idle workers

Description

This goes through a pool’s idle workers and gets rid of those that have been idle for at least IDLE_WORKER_TIMEOUT seconds.

We don’t want to disturb isolated CPUs because of a pcpu kworker being culled, so this also resets worker affinity. This requires a sleepable context, hence the split between timer callback and work item.

void maybe_create_worker(struct worker_pool *pool)¶: create a new worker if necessary

Parameters

struct worker_pool *pool: pool to create a new worker for

Description

Create a new worker for pool if necessary. pool is guaranteed to have at least one idle worker on return from this function. If creating a new worker takes longer than MAYDAY_INTERVAL, mayday is sent to all rescuers with works scheduled on pool to resolve possible allocation deadlock.

On return, need_to_create_worker() is guaranteed to be false and may_start_working() true.

LOCKING: raw_spin_lock_irq(pool->lock) which may be released and regrabbed multiple times. Does GFP_KERNEL allocations. Called only from manager.

bool manage_workers(struct worker *worker)¶: manage worker pool

Parameters

struct worker *worker: self

Description

Assume the manager role and manage the worker pool worker belongs to. At any given time, there can be only zero or one manager per pool. The exclusion is handled automatically by this function.

The caller can safely start processing works on false return. On true return, it’s guaranteed that need_to_create_worker() is false and may_start_working() is true.

Context

raw_spin_lock_irq(pool->lock) which may be released and regrabbed multiple times. Does GFP_KERNEL allocations.

Return

false if the pool doesn’t need management and the caller can safely start processing works, true if management function was performed and the conditions that the caller verified before calling the function may no longer be true.

void process_one_work(struct worker *worker, struct work_struct *work)¶: process single work

Parameters

struct worker *worker: self
struct work_struct *work: work to process

Description

Process work. This function contains all the logics necessary to process a single work including synchronization against and interaction with other workers on the same cpu, queueing and flushing. As long as context requirement is met, any worker can call this function to process a work.

Context

raw_spin_lock_irq(pool->lock) which is released and regrabbed.

void process_scheduled_works(struct worker *worker)¶: process scheduled works

Parameters

struct worker *worker: self

Description

Process all scheduled works. Please note that the scheduled list may change while processing a work, so this function repeatedly fetches a work from the top and executes it.

Context

raw_spin_lock_irq(pool->lock) which may be released and regrabbed multiple times.

int worker_thread(void *__worker)¶: the worker thread function

Parameters

void *__worker: self

Description

The worker thread function. All workers belong to a worker_pool - either a per-cpu one or dynamic unbound one. These workers process all work items regardless of their specific target workqueue. The only exception is work items which belong to workqueues with a rescuer which will be explained in rescuer_thread().

Return

0

int rescuer_thread(void *__rescuer)¶: the rescuer thread function

Parameters

void *__rescuer: self

Description

Workqueue rescuer thread function. There’s one rescuer for each workqueue which has WQ_MEM_RECLAIM set.

Regular work processing on a pool may block trying to create a new worker which uses GFP_KERNEL allocation which has slight chance of developing into deadlock if some works currently on the same queue need to be processed to satisfy the GFP_KERNEL allocation. This is the problem rescuer solves.

When such condition is possible, the pool summons rescuers of all workqueues which have works queued on the pool and let them process those works so that forward progress can be guaranteed.

This should happen rarely.

Return

0

void check_flush_dependency(struct workqueue_struct *target_wq, struct work_struct *target_work)¶: check for flush dependency sanity

Parameters

struct workqueue_struct *target_wq: workqueue being flushed
struct work_struct *target_work: work item being flushed (NULL for workqueue flushes)

Description

current is trying to flush the whole target_wq or target_work on it. If target_wq doesn’t have WQ_MEM_RECLAIM, verify that current is not reclaiming memory or running on a workqueue which doesn’t have WQ_MEM_RECLAIM as that can break forward-progress guarantee leading to a deadlock.

void insert_wq_barrier(struct pool_workqueue *pwq, struct wq_barrier *barr, struct work_struct *target, struct worker *worker)¶: insert a barrier work

Parameters

struct pool_workqueue *pwq: pwq to insert barrier into
struct wq_barrier *barr: wq_barrier to insert
struct work_struct *target: target work to attach barr to
struct worker *worker: worker currently executing target, NULL if target is not executing

Description

barr is linked to target such that barr is completed only after target finishes execution. Please note that the ordering guarantee is observed only with respect to target and on the local cpu.

Currently, a queued barrier can’t be canceled. This is because try_to_grab_pending() can’t determine whether the work to be grabbed is at the head of the queue and thus can’t clear LINKED flag of the previous work while there must be a valid next work after a work with LINKED flag set.

Note that when worker is non-NULL, target may be modified underneath us, so we can’t reliably determine pwq from target.

Context

raw_spin_lock_irq(pool->lock).

bool flush_workqueue_prep_pwqs(struct workqueue_struct *wq, int flush_color, int work_color)¶: prepare pwqs for workqueue flushing

Parameters

struct workqueue_struct *wq: workqueue being flushed
int flush_color: new flush color, < 0 for no-op
int work_color: new work color, < 0 for no-op

Description

Prepare pwqs for workqueue flushing.

If flush_color is non-negative, flush_color on all pwqs should be -1. If no pwq has in-flight commands at the specified color, all pwq->flush_color’s stay at -1 and false is returned. If any pwq has in flight commands, its pwq->flush_color is set to flush_color, wq->nr_pwqs_to_flush is updated accordingly, pwq wakeup logic is armed and true is returned.

The caller should have initialized wq->first_flusher prior to calling this function with non-negative flush_color. If flush_color is negative, no flush color update is done and false is returned.

If work_color is non-negative, all pwqs should have the same work_color which is previous to work_color and all will be advanced to work_color.

Context

mutex_lock(wq->mutex).

Return

true if flush_color >= 0 and there’s something to flush. false otherwise.

void __flush_workqueue(struct workqueue_struct *wq)¶: ensure that any scheduled work has run to completion.

Parameters

struct workqueue_struct *wq: workqueue to flush

Description

This function sleeps until all work items which were queued on entry have finished execution, but it is not livelocked by new incoming ones.

void drain_workqueue(struct workqueue_struct *wq)¶: drain a workqueue

Parameters

struct workqueue_struct *wq: workqueue to drain

Description

Wait until the workqueue becomes empty. While draining is in progress, only chain queueing is allowed. IOW, only currently pending or running work items on wq can queue further work items on it. wq is flushed repeatedly until it becomes empty. The number of flushing is determined by the depth of chaining and should be relatively short. Whine if it takes too long.

bool flush_work(struct work_struct *work)¶: wait for a work to finish executing the last queueing instance

Parameters

struct work_struct *work: the work to flush

Description

Wait until work has finished execution. work is guaranteed to be idle on return if it hasn’t been requeued since flush started.

Return

true if flush_work() waited for the work to finish execution, false if it was already idle.

bool flush_delayed_work(struct delayed_work *dwork)¶: wait for a dwork to finish executing the last queueing

Parameters

struct delayed_work *dwork: the delayed work to flush

Description

Delayed timer is cancelled and the pending work is queued for immediate execution. Like flush_work(), this function only considers the last queueing instance of dwork.

Return

true if flush_work() waited for the work to finish execution, false if it was already idle.

bool flush_rcu_work(struct rcu_work *rwork)¶: wait for a rwork to finish executing the last queueing

Parameters

struct rcu_work *rwork: the rcu work to flush

Return

true if flush_rcu_work() waited for the work to finish execution, false if it was already idle.

bool cancel_work_sync(struct work_struct *work)¶: cancel a work and wait for it to finish

Parameters

struct work_struct *work: the work to cancel

Description

Cancel work and wait for its execution to finish. This function can be used even if the work re-queues itself or migrates to another workqueue. On return from this function, work is guaranteed to be not pending or executing on any CPU as long as there aren’t racing enqueues.

cancel_work_sync(delayed_work->work) must not be used for delayed_work’s. Use cancel_delayed_work_sync() instead.

Must be called from a sleepable context if work was last queued on a non-BH workqueue. Can also be called from non-hardirq atomic contexts including BH if work was last queued on a BH workqueue.

Returns true if work was pending, false otherwise.

bool cancel_delayed_work(struct delayed_work *dwork)¶: cancel a delayed work

Parameters

struct delayed_work *dwork: delayed_work to cancel

Description

Kill off a pending delayed_work.

This function is safe to call from any context including IRQ handler.

Return

true if dwork was pending and canceled; false if it wasn’t pending.

Note

The work callback function may still be running on return, unless it returns true and the work doesn’t re-arm itself. Explicitly flush or use cancel_delayed_work_sync() to wait on it.

bool cancel_delayed_work_sync(struct delayed_work *dwork)¶: cancel a delayed work and wait for it to finish

Parameters

struct delayed_work *dwork: the delayed work cancel

Description

This is cancel_work_sync() for delayed works.

Return

true if dwork was pending, false otherwise.

bool disable_work(struct work_struct *work)¶: Disable and cancel a work item

Parameters

struct work_struct *work: work item to disable

Description

Disable work by incrementing its disable count and cancel it if currently pending. As long as the disable count is non-zero, any attempt to queue work will fail and return false. The maximum supported disable depth is 2 to the power of WORK_OFFQ_DISABLE_BITS, currently 65536.

Can be called from any context. Returns true if work was pending, false otherwise.

bool disable_work_sync(struct work_struct *work)¶: Disable, cancel and drain a work item

Parameters

struct work_struct *work: work item to disable

Description

Similar to disable_work() but also wait for work to finish if currently executing.

Must be called from a sleepable context if work was last queued on a non-BH workqueue. Can also be called from non-hardirq atomic contexts including BH if work was last queued on a BH workqueue.

Returns true if work was pending, false otherwise.

bool enable_work(struct work_struct *work)¶: Enable a work item

Parameters

struct work_struct *work: work item to enable

Description

Undo disable_work[_sync]() by decrementing work’s disable count. work can only be queued if its disable count is 0.

Can be called from any context. Returns true if the disable count reached 0. Otherwise, false.

bool disable_delayed_work(struct delayed_work *dwork)¶: Disable and cancel a delayed work item

Parameters

struct delayed_work *dwork: delayed work item to disable

Description

disable_work() for delayed work items.

bool disable_delayed_work_sync(struct delayed_work *dwork)¶: Disable, cancel and drain a delayed work item

Parameters

struct delayed_work *dwork: delayed work item to disable

Description

disable_work_sync() for delayed work items.

bool enable_delayed_work(struct delayed_work *dwork)¶: Enable a delayed work item

Parameters

struct delayed_work *dwork: delayed work item to enable

Description

enable_work() for delayed work items.

int schedule_on_each_cpu(work_func_t func)¶: execute a function synchronously on each online CPU

Parameters

work_func_t func: the function to call

Description

schedule_on_each_cpu() executes func on each online CPU using the system workqueue and blocks until all CPUs have completed. schedule_on_each_cpu() is very slow.

Return

0 on success, -errno on failure.

int execute_in_process_context(work_func_t fn, struct execute_work *ew)¶: reliably execute the routine with user context

Parameters

work_func_t fn: the function to execute
struct execute_work *ew: guaranteed storage for the execute work structure (must be available when the work executes)

Description

Executes the function immediately if process context is available, otherwise schedules the function for delayed execution.

Return

0 - function was executed: 1 - function was scheduled for execution

void free_workqueue_attrs(struct workqueue_attrs *attrs)¶: free a workqueue_attrs

Parameters

struct workqueue_attrs *attrs: workqueue_attrs to free

Description

Undo alloc_workqueue_attrs().

struct workqueue_attrs *alloc_workqueue_attrs(void)¶: allocate a workqueue_attrs

Parameters

void: no arguments

Description

Allocate a new workqueue_attrs, initialize with default settings and return it.

Return

The allocated new workqueue_attr on success. NULL on failure.

int init_worker_pool(struct worker_pool *pool)¶: initialize a newly zalloc’d worker_pool

Parameters

struct worker_pool *pool: worker_pool to initialize

Description

Initialize a newly zalloc’d pool. It also allocates pool->attrs.

Return

0 on success, -errno on failure. Even on failure, all fields inside pool proper are initialized and put_unbound_pool() can be called on pool safely to release it.

void put_unbound_pool(struct worker_pool *pool)¶: put a worker_pool

Parameters

struct worker_pool *pool: worker_pool to put

Description

Put pool. If its refcnt reaches zero, it gets destroyed in RCU safe manner. get_unbound_pool() calls this function on its failure path and this function should be able to release pools which went through, successfully or not, init_worker_pool().

Should be called with wq_pool_mutex held.

struct worker_pool *get_unbound_pool(const struct workqueue_attrs *attrs)¶: get a worker_pool with the specified attributes

Parameters

const struct workqueue_attrs *attrs: the attributes of the worker_pool to get

Description

Obtain a worker_pool which has the same attributes as attrs, bump the reference count and return it. If there already is a matching worker_pool, it will be used; otherwise, this function attempts to create a new one.

Should be called with wq_pool_mutex held.

Return

On success, a worker_pool with the same attributes as attrs. On failure, NULL.

void wq_calc_pod_cpumask(struct workqueue_attrs *attrs, int cpu, int cpu_going_down)¶: calculate a wq_attrs’ cpumask for a pod

Parameters

struct workqueue_attrs *attrs: the wq_attrs of the default pwq of the target workqueue
int cpu: the target CPU
int cpu_going_down: if >= 0, the CPU to consider as offline

Description

Calculate the cpumask a workqueue with attrs should use on pod. If cpu_going_down is >= 0, that cpu is considered offline during calculation. The result is stored in attrs->__pod_cpumask.

If pod affinity is not enabled, attrs->cpumask is always used. If enabled and pod has online CPUs requested by attrs, the returned cpumask is the intersection of the possible CPUs of pod and attrs->cpumask.

The caller is responsible for ensuring that the cpumask of pod stays stable.

int apply_workqueue_attrs(struct workqueue_struct *wq, const struct workqueue_attrs *attrs)¶: apply new workqueue_attrs to an unbound workqueue

Parameters

struct workqueue_struct *wq: the target workqueue
const struct workqueue_attrs *attrs: the workqueue_attrs to apply, allocated with alloc_workqueue_attrs()

Description

Apply attrs to an unbound workqueue wq. Unless disabled, this function maps a separate pwq to each CPU pod with possibles CPUs in attrs->cpumask so that work items are affine to the pod it was issued on. Older pwqs are released as in-flight work items finish. Note that a work item which repeatedly requeues itself back-to-back will stay on its current pwq.

Performs GFP_KERNEL allocations.

Assumes caller has CPU hotplug read exclusion, i.e. cpus_read_lock().

Return

0 on success and -errno on failure.

void wq_update_pod(struct workqueue_struct *wq, int cpu, int hotplug_cpu, bool online)¶: update pod affinity of a wq for CPU hot[un]plug

Parameters

struct workqueue_struct *wq: the target workqueue
int cpu: the CPU to update pool association for
int hotplug_cpu: the CPU coming up or going down
bool online: whether cpu is coming up or going down

Description

This function is to be called from CPU_DOWN_PREPARE, CPU_ONLINE and CPU_DOWN_FAILED. cpu is being hot[un]plugged, update pod affinity of wq accordingly.

If pod affinity can’t be adjusted due to memory allocation failure, it falls back to wq->dfl_pwq which may not be optimal but is always correct.

Note that when the last allowed CPU of a pod goes offline for a workqueue with a cpumask spanning multiple pods, the workers which were already executing the work items for the workqueue will lose their CPU affinity and may execute on any CPU. This is similar to how per-cpu workqueues behave on CPU_DOWN. If a workqueue user wants strict affinity, it’s the user’s responsibility to flush the work item from CPU_DOWN_PREPARE.

void wq_adjust_max_active(struct workqueue_struct *wq)¶: update a wq’s max_active to the current setting

Parameters

struct workqueue_struct *wq: target workqueue

Description

If wq isn’t freezing, set wq->max_active to the saved_max_active and activate inactive work items accordingly. If wq is freezing, clear wq->max_active to zero.

void destroy_workqueue(struct workqueue_struct *wq)¶: safely terminate a workqueue

Parameters

struct workqueue_struct *wq: target workqueue

Description

Safely destroy a workqueue. All work currently pending will be done first.

void workqueue_set_max_active(struct workqueue_struct *wq, int max_active)¶: adjust max_active of a workqueue

Parameters

struct workqueue_struct *wq: target workqueue
int max_active: new max_active value.

Description

Set max_active of wq to max_active. See the alloc_workqueue() function comment.

Context

Don’t call from IRQ context.

void workqueue_set_min_active(struct workqueue_struct *wq, int min_active)¶: adjust min_active of an unbound workqueue

Parameters

struct workqueue_struct *wq: target unbound workqueue
int min_active: new min_active value

Description

Set min_active of an unbound workqueue. Unlike other types of workqueues, an unbound workqueue is not guaranteed to be able to process max_active interdependent work items. Instead, an unbound workqueue is guaranteed to be able to process min_active number of interdependent work items which is WQ_DFL_MIN_ACTIVE by default.

Use this function to adjust the min_active value between 0 and the current max_active.

struct work_struct *current_work(void)¶: retrieve current task’s work struct

Parameters

void: no arguments

Description

Determine if current task is a workqueue worker and what it’s working on. Useful to find out the context that the current task is running in.

Return

work struct if current task is a workqueue worker, NULL otherwise.

bool current_is_workqueue_rescuer(void)¶: is current workqueue rescuer?

Parameters

void: no arguments

Description

Determine whether current is a workqueue rescuer. Can be used from work functions to determine whether it’s being run off the rescuer task.

Return

true if current is a workqueue rescuer. false otherwise.

bool workqueue_congested(int cpu, struct workqueue_struct *wq)¶: test whether a workqueue is congested

Parameters

int cpu: CPU in question
struct workqueue_struct *wq: target workqueue

Description

Test whether wq’s cpu workqueue for cpu is congested. There is no synchronization around this function and the test result is unreliable and only useful as advisory hints or for debugging.

If cpu is WORK_CPU_UNBOUND, the test is performed on the local CPU.

With the exception of ordered workqueues, all workqueues have per-cpu pool_workqueues, each with its own congested state. A workqueue being congested on one CPU doesn’t mean that the workqueue is contested on any other CPUs.

Return

true if congested, false otherwise.

unsigned int work_busy(struct work_struct *work)¶: test whether a work is currently pending or running

Parameters

struct work_struct *work: the work to be tested

Description

Test whether work is currently pending or running. There is no synchronization around this function and the test result is unreliable and only useful as advisory hints or for debugging.

Return

OR’d bitmask of WORK_BUSY_* bits.

void set_worker_desc(const char *fmt, ...)¶: set description for the current work item

Parameters

const char *fmt: printf-style format string
...: arguments for the format string

Description

This function can be called by a running work function to describe what the work item is about. If the worker task gets dumped, this information will be printed out together to help debugging. The description can be at most WORKER_DESC_LEN including the trailing ‘0’.

void print_worker_info(const char *log_lvl, struct task_struct *task)¶: print out worker information and description

Parameters

const char *log_lvl: the log level to use when printing
struct task_struct *task: target task

Description

If task is a worker and currently executing a work item, print out the name of the workqueue being serviced and worker description set with set_worker_desc() by the currently executing work item.

This function can be safely called on any task as long as the task_struct itself is accessible. While safe, this function isn’t synchronized and may print out mixups or garbages of limited length.

void show_one_workqueue(struct workqueue_struct *wq)¶: dump state of specified workqueue

Parameters

struct workqueue_struct *wq: workqueue whose state will be printed

void show_one_worker_pool(struct worker_pool *pool)¶: dump state of specified worker pool

Parameters

struct worker_pool *pool: worker pool whose state will be printed

void show_all_workqueues(void)¶: dump workqueue state

Parameters

void: no arguments

Description

Called from a sysrq handler and prints out all busy workqueues and pools.

void show_freezable_workqueues(void)¶: dump freezable workqueue state

Parameters

void: no arguments

Description

Called from try_to_freeze_tasks() and prints out all freezable workqueues still busy.

void rebind_workers(struct worker_pool *pool)¶: rebind all workers of a pool to the associated CPU

Parameters

struct worker_pool *pool: pool of interest

Description

pool->cpu is coming online. Rebind all workers to the CPU.

void restore_unbound_workers_cpumask(struct worker_pool *pool, int cpu)¶: restore cpumask of unbound workers

Parameters

struct worker_pool *pool: unbound pool of interest
int cpu: the CPU which is coming up

Description

An unbound pool may end up with a cpumask which doesn’t have any online CPUs. When a worker of such pool get scheduled, the scheduler resets its cpus_allowed. If cpu is in pool’s cpumask which didn’t have any online CPU before, cpus_allowed of all its workers should be restored.

long work_on_cpu_key(int cpu, long (*fn)(void*), void *arg, struct lock_class_key *key)¶: run a function in thread context on a particular cpu

Parameters

int cpu: the cpu to run on
long (*fn)(void *): the function to run
void *arg: the function arg
struct lock_class_key *key: The lock class key for lock debugging purposes

Description

It is up to the caller to ensure that the cpu doesn’t go offline. The caller must not hold any locks which would prevent fn from completing.

Return

The value fn returns.

long work_on_cpu_safe_key(int cpu, long (*fn)(void*), void *arg, struct lock_class_key *key)¶: run a function in thread context on a particular cpu

Parameters

int cpu: the cpu to run on
long (*fn)(void *): the function to run
void *arg: the function argument
struct lock_class_key *key: The lock class key for lock debugging purposes

Description

Disables CPU hotplug and calls work_on_cpu(). The caller must not hold any locks which would prevent fn from completing.

Return

The value fn returns.

void freeze_workqueues_begin(void)¶: begin freezing workqueues

Parameters

void: no arguments

Description

Start freezing workqueues. After this function returns, all freezable workqueues will queue new works to their inactive_works list instead of pool->worklist.

Context

Grabs and releases wq_pool_mutex, wq->mutex and pool->lock’s.

bool freeze_workqueues_busy(void)¶: are freezable workqueues still busy?

Parameters

void: no arguments

Description

Check whether freezing is complete. This function must be called between freeze_workqueues_begin() and thaw_workqueues().

Context

Grabs and releases wq_pool_mutex.

Return

true if some freezable workqueues are still busy. false if freezing is complete.

void thaw_workqueues(void)¶: thaw workqueues

Parameters

void: no arguments

Description

Thaw workqueues. Normal queueing is restored and all collected frozen works are transferred to their respective pool worklists.

Context

Grabs and releases wq_pool_mutex, wq->mutex and pool->lock’s.

int workqueue_unbound_exclude_cpumask(cpumask_var_t exclude_cpumask)¶: Exclude given CPUs from unbound cpumask

Parameters

cpumask_var_t exclude_cpumask: the cpumask to be excluded from wq_unbound_cpumask

Description

This function can be called from cpuset code to provide a set of isolated CPUs that should be excluded from wq_unbound_cpumask. The caller must hold either cpus_read_lock or cpus_write_lock.

int workqueue_set_unbound_cpumask(cpumask_var_t cpumask)¶: Set the low-level unbound cpumask

Parameters

cpumask_var_t cpumask

the cpumask to set

The low-level workqueues cpumask is a global cpumask that limits the affinity of all unbound workqueues. This function check the cpumask and apply it to all unbound workqueues and updates all pwqs of them.

Return

0 - Success: -EINVAL - Invalid cpumask -ENOMEM - Failed to allocate memory for attrs or pwqs.

int workqueue_sysfs_register(struct workqueue_struct *wq)¶: make a workqueue visible in sysfs

Parameters

struct workqueue_struct *wq: the workqueue to register

Description

Expose wq in sysfs under /sys/bus/workqueue/devices. alloc_workqueue*() automatically calls this function if WQ_SYSFS is set which is the preferred method.

Workqueue user should use this function directly iff it wants to apply workqueue_attrs before making the workqueue visible in sysfs; otherwise, apply_workqueue_attrs() may race against userland updating the attributes.

Return

0 on success, -errno on failure.

void workqueue_sysfs_unregister(struct workqueue_struct *wq)¶: undo workqueue_sysfs_register()

Parameters

struct workqueue_struct *wq: the workqueue to unregister

Description

If wq is registered to sysfs by workqueue_sysfs_register(), unregister.

void workqueue_init_early(void)¶: early init for workqueue subsystem

Parameters

void: no arguments

Description

This is the first step of three-staged workqueue subsystem initialization and invoked as soon as the bare basics - memory allocation, cpumasks and idr are up. It sets up all the data structures and system workqueues and allows early boot code to create workqueues and queue/cancel work items. Actual work item execution starts only after kthreads can be created and scheduled right before early initcalls.

void workqueue_init(void)¶: bring workqueue subsystem fully online

Parameters

void: no arguments

Description

This is the second step of three-staged workqueue subsystem initialization and invoked as soon as kthreads can be created and scheduled. Workqueues have been created and work items queued on them, but there are no kworkers executing the work items yet. Populate the worker pools with the initial workers and enable future kworker creations.

void workqueue_init_topology(void)¶: initialize CPU pods for unbound workqueues

Parameters

void: no arguments

Description

This is the third step of three-staged workqueue subsystem initialization and invoked after SMP and topology information are fully initialized. It initializes the unbound CPU pods accordingly.

The Linux Kernel

Contents

This Page

Workqueue¶

Introduction¶

Why Concurrency Managed Workqueue?¶

The Design¶

Application Programming Interface (API)¶

`flags`¶

`max_active`¶

Example Execution Scenarios¶

Guidelines¶

Affinity Scopes¶

Affinity Scopes and Performance¶

Scenario 1: Enough issuers and work spread across the machine¶

Scenario 2: Fewer issuers, enough work for saturation¶

Scenario 3: Even fewer issuers, not enough work to saturate¶

Conclusion and Recommendations¶

Examining Configuration¶

Monitoring¶

Debugging¶

Non-reentrance Conditions¶

Kernel Inline Documentations Reference¶

1	if work was pending and we successfully stole PENDING
0	if work was idle and we claimed PENDING
-EAGAIN	if PENDING couldn’t be grabbed at the moment, safe to busy-retry

Workqueue¶

Introduction¶

Why Concurrency Managed Workqueue?¶

The Design¶

Application Programming Interface (API)¶

flags¶

max_active¶

Example Execution Scenarios¶

Guidelines¶

Affinity Scopes¶

Affinity Scopes and Performance¶

Scenario 1: Enough issuers and work spread across the machine¶

Scenario 2: Fewer issuers, enough work for saturation¶

Scenario 3: Even fewer issuers, not enough work to saturate¶

Conclusion and Recommendations¶

Examining Configuration¶

Monitoring¶

Debugging¶

Non-reentrance Conditions¶

Kernel Inline Documentations Reference¶

`flags`¶

`max_active`¶