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yaml
---
r: 316715
b: refs/heads/master
c: 3270476
h: refs/heads/master
i:
  316713: c045bdc
  316711: b3c9645
v: v3
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Tejun Heo committed Jul 14, 2012
1 parent a927050 commit bcb9e0f
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2 changes: 1 addition & 1 deletion [refs]
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@@ -1,2 +1,2 @@
---
refs/heads/master: 4ce62e9e30cacc26885cab133ad1de358dd79f21
refs/heads/master: 3270476a6c0ce322354df8679652f060d66526dc
103 changes: 38 additions & 65 deletions trunk/Documentation/workqueue.txt
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Expand Up @@ -89,52 +89,55 @@ called thread-pools.

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

The backend is called gcwq. There is one gcwq for each possible CPU
and one gcwq to serve work items queued on unbound workqueues.
and one gcwq to serve work items queued on unbound workqueues. Each
gcwq has two thread-pools - one for normal work items and the other
for high priority ones.

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, reentrancy, concurrency limits and more. To
get a detailed overview refer to the API description of
things like CPU locality, reentrancy, 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 gcwq is
determined according to the queue parameters and workqueue attributes
and appended on the shared worklist of the gcwq. For example, unless
specifically overridden, a work item of a bound workqueue will be
queued on the worklist of exactly that gcwq that is associated to the
CPU the issuer is running on.
When a work item is queued to a workqueue, the target gcwq and
thread-pool is determined according to the queue parameters and
workqueue attributes and appended on the shared worklist of the
thread-pool. For example, unless specifically overridden, a work item
of a bound workqueue will be queued on the worklist of either normal
or highpri thread-pool of the gcwq that is associated to the CPU the
issuer is running on.

For any worker 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 gcwq bound to an actual CPU implements concurrency management by
hooking into the scheduler. The gcwq 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 gcwq 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.
Each thread-pool bound to an actual CPU implements concurrency
management by hooking into the scheduler. The thread-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 thread-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 an unbound wq, the above concurrency management doesn't apply and
the gcwq for the pseudo unbound CPU tries to start executing all work
items as soon as possible. The responsibility of regulating
the thread-pools for the pseudo unbound CPU try to start executing all
work items as soon as possible. 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.
Expand Down Expand Up @@ -205,31 +208,22 @@ resources, scheduled and executed.

WQ_HIGHPRI

Work items of a highpri wq are queued at the head of the
worklist of the target gcwq and start execution regardless of
the current concurrency level. In other words, highpri work
items will always start execution as soon as execution
resource is available.
Work items of a highpri wq are queued to the highpri
thread-pool of the target gcwq. Highpri thread-pools are
served by worker threads with elevated nice level.

Ordering among highpri work items is preserved - a highpri
work item queued after another highpri work item will start
execution after the earlier highpri work item starts.

Although highpri work items are not held back by other
runnable work items, they still contribute to the concurrency
level. Highpri work items in runnable state will prevent
non-highpri work items from starting execution.

This flag is meaningless for unbound wq.
Note that normal and highpri thread-pools don't interact with
each other. Each maintain 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 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.
work items will not prevent other work items in the same
thread-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
Expand All @@ -239,14 +233,6 @@ resources, scheduled and executed.

This flag is meaningless for unbound wq.

WQ_HIGHPRI | WQ_CPU_INTENSIVE

This combination makes the wq avoid interaction with
concurrency management completely and behave as a simple
per-CPU execution context provider. Work items queued on a
highpri CPU-intensive wq start execution as soon as resources
are available and don't affect execution of other work items.

@max_active:

@max_active determines the maximum number of execution contexts per
Expand Down Expand Up @@ -328,20 +314,7 @@ If @max_active == 2,
35 w2 wakes up and finishes

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

TIME IN MSECS EVENT
0 w1 and w2 start and burn CPU
5 w1 sleeps
10 w2 sleeps
10 w0 starts and burns CPU
15 w0 sleeps
15 w1 wakes up and finishes
20 w2 wakes up and finishes
25 w0 wakes up and burns CPU
30 w0 finishes

If q1 has WQ_CPU_INTENSIVE set,
WQ_CPU_INTENSIVE set,

TIME IN MSECS EVENT
0 w0 starts and burns CPU
Expand Down
100 changes: 27 additions & 73 deletions trunk/kernel/workqueue.c
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Original file line number Diff line number Diff line change
Expand Up @@ -52,7 +52,6 @@ enum {
/* pool flags */
POOL_MANAGE_WORKERS = 1 << 0, /* need to manage workers */
POOL_MANAGING_WORKERS = 1 << 1, /* managing workers */
POOL_HIGHPRI_PENDING = 1 << 2, /* highpri works on queue */

/* worker flags */
WORKER_STARTED = 1 << 0, /* started */
Expand All @@ -74,7 +73,7 @@ enum {
TRUSTEE_RELEASE = 3, /* release workers */
TRUSTEE_DONE = 4, /* trustee is done */

NR_WORKER_POOLS = 1, /* # worker pools per gcwq */
NR_WORKER_POOLS = 2, /* # worker pools per gcwq */

BUSY_WORKER_HASH_ORDER = 6, /* 64 pointers */
BUSY_WORKER_HASH_SIZE = 1 << BUSY_WORKER_HASH_ORDER,
Expand All @@ -95,6 +94,7 @@ enum {
* all cpus. Give -20.
*/
RESCUER_NICE_LEVEL = -20,
HIGHPRI_NICE_LEVEL = -20,
};

/*
Expand Down Expand Up @@ -174,7 +174,7 @@ struct global_cwq {
struct hlist_head busy_hash[BUSY_WORKER_HASH_SIZE];
/* L: hash of busy workers */

struct worker_pool pool; /* the worker pools */
struct worker_pool pools[2]; /* normal and highpri pools */

struct task_struct *trustee; /* L: for gcwq shutdown */
unsigned int trustee_state; /* L: trustee state */
Expand Down Expand Up @@ -277,7 +277,8 @@ EXPORT_SYMBOL_GPL(system_nrt_freezable_wq);
#include <trace/events/workqueue.h>

#define for_each_worker_pool(pool, gcwq) \
for ((pool) = &(gcwq)->pool; (pool); (pool) = NULL)
for ((pool) = &(gcwq)->pools[0]; \
(pool) < &(gcwq)->pools[NR_WORKER_POOLS]; (pool)++)

#define for_each_busy_worker(worker, i, pos, gcwq) \
for (i = 0; i < BUSY_WORKER_HASH_SIZE; i++) \
Expand Down Expand Up @@ -473,6 +474,11 @@ static atomic_t unbound_pool_nr_running[NR_WORKER_POOLS] = {

static int worker_thread(void *__worker);

static int worker_pool_pri(struct worker_pool *pool)
{
return pool - pool->gcwq->pools;
}

static struct global_cwq *get_gcwq(unsigned int cpu)
{
if (cpu != WORK_CPU_UNBOUND)
Expand All @@ -484,7 +490,7 @@ static struct global_cwq *get_gcwq(unsigned int cpu)
static atomic_t *get_pool_nr_running(struct worker_pool *pool)
{
int cpu = pool->gcwq->cpu;
int idx = 0;
int idx = worker_pool_pri(pool);

if (cpu != WORK_CPU_UNBOUND)
return &per_cpu(pool_nr_running, cpu)[idx];
Expand Down Expand Up @@ -586,15 +592,14 @@ static struct global_cwq *get_work_gcwq(struct work_struct *work)
}

/*
* Policy functions. These define the policies on how the global
* worker pool is managed. Unless noted otherwise, these functions
* assume that they're being called with gcwq->lock held.
* Policy functions. These define the policies on how the global worker
* pools are managed. Unless noted otherwise, these functions assume that
* they're being called with gcwq->lock held.
*/

static bool __need_more_worker(struct worker_pool *pool)
{
return !atomic_read(get_pool_nr_running(pool)) ||
(pool->flags & POOL_HIGHPRI_PENDING);
return !atomic_read(get_pool_nr_running(pool));
}

/*
Expand All @@ -621,9 +626,7 @@ static bool keep_working(struct worker_pool *pool)
{
atomic_t *nr_running = get_pool_nr_running(pool);

return !list_empty(&pool->worklist) &&
(atomic_read(nr_running) <= 1 ||
(pool->flags & POOL_HIGHPRI_PENDING));
return !list_empty(&pool->worklist) && atomic_read(nr_running) <= 1;
}

/* Do we need a new worker? Called from manager. */
Expand Down Expand Up @@ -891,43 +894,6 @@ static struct worker *find_worker_executing_work(struct global_cwq *gcwq,
work);
}

/**
* pool_determine_ins_pos - find insertion position
* @pool: pool of interest
* @cwq: cwq a work is being queued for
*
* A work for @cwq is about to be queued on @pool, determine insertion
* position for the work. If @cwq is for HIGHPRI wq, the work is
* queued at the head of the queue but in FIFO order with respect to
* other HIGHPRI works; otherwise, at the end of the queue. This
* function also sets POOL_HIGHPRI_PENDING flag to hint @pool that
* there are HIGHPRI works pending.
*
* CONTEXT:
* spin_lock_irq(gcwq->lock).
*
* RETURNS:
* Pointer to inserstion position.
*/
static inline struct list_head *pool_determine_ins_pos(struct worker_pool *pool,
struct cpu_workqueue_struct *cwq)
{
struct work_struct *twork;

if (likely(!(cwq->wq->flags & WQ_HIGHPRI)))
return &pool->worklist;

list_for_each_entry(twork, &pool->worklist, entry) {
struct cpu_workqueue_struct *tcwq = get_work_cwq(twork);

if (!(tcwq->wq->flags & WQ_HIGHPRI))
break;
}

pool->flags |= POOL_HIGHPRI_PENDING;
return &twork->entry;
}

/**
* insert_work - insert a work into gcwq
* @cwq: cwq @work belongs to
Expand Down Expand Up @@ -1068,7 +1034,7 @@ static void __queue_work(unsigned int cpu, struct workqueue_struct *wq,
if (likely(cwq->nr_active < cwq->max_active)) {
trace_workqueue_activate_work(work);
cwq->nr_active++;
worklist = pool_determine_ins_pos(cwq->pool, cwq);
worklist = &cwq->pool->worklist;
} else {
work_flags |= WORK_STRUCT_DELAYED;
worklist = &cwq->delayed_works;
Expand Down Expand Up @@ -1385,6 +1351,7 @@ static struct worker *create_worker(struct worker_pool *pool, bool bind)
{
struct global_cwq *gcwq = pool->gcwq;
bool on_unbound_cpu = gcwq->cpu == WORK_CPU_UNBOUND;
const char *pri = worker_pool_pri(pool) ? "H" : "";
struct worker *worker = NULL;
int id = -1;

Expand All @@ -1406,15 +1373,17 @@ static struct worker *create_worker(struct worker_pool *pool, bool bind)

if (!on_unbound_cpu)
worker->task = kthread_create_on_node(worker_thread,
worker,
cpu_to_node(gcwq->cpu),
"kworker/%u:%d", gcwq->cpu, id);
worker, cpu_to_node(gcwq->cpu),
"kworker/%u:%d%s", gcwq->cpu, id, pri);
else
worker->task = kthread_create(worker_thread, worker,
"kworker/u:%d", id);
"kworker/u:%d%s", id, pri);
if (IS_ERR(worker->task))
goto fail;

if (worker_pool_pri(pool))
set_user_nice(worker->task, HIGHPRI_NICE_LEVEL);

/*
* A rogue worker will become a regular one if CPU comes
* online later on. Make sure every worker has
Expand Down Expand Up @@ -1761,10 +1730,9 @@ static void cwq_activate_first_delayed(struct cpu_workqueue_struct *cwq)
{
struct work_struct *work = list_first_entry(&cwq->delayed_works,
struct work_struct, entry);
struct list_head *pos = pool_determine_ins_pos(cwq->pool, cwq);

trace_workqueue_activate_work(work);
move_linked_works(work, pos, NULL);
move_linked_works(work, &cwq->pool->worklist, NULL);
__clear_bit(WORK_STRUCT_DELAYED_BIT, work_data_bits(work));
cwq->nr_active++;
}
Expand Down Expand Up @@ -1879,21 +1847,6 @@ __acquires(&gcwq->lock)
set_work_cpu(work, gcwq->cpu);
list_del_init(&work->entry);

/*
* If HIGHPRI_PENDING, check the next work, and, if HIGHPRI,
* wake up another worker; otherwise, clear HIGHPRI_PENDING.
*/
if (unlikely(pool->flags & POOL_HIGHPRI_PENDING)) {
struct work_struct *nwork = list_first_entry(&pool->worklist,
struct work_struct, entry);

if (!list_empty(&pool->worklist) &&
get_work_cwq(nwork)->wq->flags & WQ_HIGHPRI)
wake_up_worker(pool);
else
pool->flags &= ~POOL_HIGHPRI_PENDING;
}

/*
* CPU intensive works don't participate in concurrency
* management. They're the scheduler's responsibility.
Expand Down Expand Up @@ -3047,9 +3000,10 @@ struct workqueue_struct *__alloc_workqueue_key(const char *fmt,
for_each_cwq_cpu(cpu, wq) {
struct cpu_workqueue_struct *cwq = get_cwq(cpu, wq);
struct global_cwq *gcwq = get_gcwq(cpu);
int pool_idx = (bool)(flags & WQ_HIGHPRI);

BUG_ON((unsigned long)cwq & WORK_STRUCT_FLAG_MASK);
cwq->pool = &gcwq->pool;
cwq->pool = &gcwq->pools[pool_idx];
cwq->wq = wq;
cwq->flush_color = -1;
cwq->max_active = max_active;
Expand Down

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