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workqueue: Move pwq->max_active to wq->max_active
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max_active is a workqueue-wide setting and the configured value is stored in
wq->saved_max_active; however, the effective value was stored in
pwq->max_active. While this is harmless, it makes max_active update process
more complicated and gets in the way of the planned max_active semantic
updates for unbound workqueues.

This patches moves pwq->max_active to wq->max_active. This simplifies the
code and makes freezing and noop max_active updates cheaper too. No
user-visible behavior change is intended.

As wq->max_active is updated while holding wq mutex but read without any
locking, it now uses WRITE/READ_ONCE(). A new locking locking rule WO is
added for it.

v2: wq->max_active now uses WRITE/READ_ONCE() as suggested by Lai.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <jiangshanlai@gmail.com>
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@htejun
htejun committed Jan 29, 2024
1 parent e563d0a commit a045a27
Showing 1 changed file with 66 additions and 67 deletions.
133 changes: 66 additions & 67 deletions kernel/workqueue.c
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Original file line number Diff line number Diff line change
Expand Up @@ -147,6 +147,9 @@ enum wq_internal_consts {
*
* WR: wq->mutex protected for writes. RCU protected for reads.
*
* WO: wq->mutex protected for writes. Updated with WRITE_ONCE() and can be read
* with READ_ONCE() without locking.
*
* MD: wq_mayday_lock protected.
*
* WD: Used internally by the watchdog.
Expand Down Expand Up @@ -254,7 +257,6 @@ struct pool_workqueue {
* is marked with WORK_STRUCT_INACTIVE iff it is in pwq->inactive_works.
*/
int nr_active; /* L: nr of active works */
int max_active; /* L: max active works */
struct list_head inactive_works; /* L: inactive works */
struct list_head pwqs_node; /* WR: node on wq->pwqs */
struct list_head mayday_node; /* MD: node on wq->maydays */
Expand Down Expand Up @@ -302,7 +304,8 @@ struct workqueue_struct {
struct worker *rescuer; /* MD: rescue worker */

int nr_drainers; /* WQ: drain in progress */
int saved_max_active; /* WQ: saved pwq max_active */
int max_active; /* WO: max active works */
int saved_max_active; /* WQ: saved max_active */

struct workqueue_attrs *unbound_attrs; /* PW: only for unbound wqs */
struct pool_workqueue *dfl_pwq; /* PW: only for unbound wqs */
Expand Down Expand Up @@ -1496,7 +1499,7 @@ static void pwq_dec_nr_in_flight(struct pool_workqueue *pwq, unsigned long work_
pwq->nr_active--;
if (!list_empty(&pwq->inactive_works)) {
/* one down, submit an inactive one */
if (pwq->nr_active < pwq->max_active)
if (pwq->nr_active < READ_ONCE(pwq->wq->max_active))
pwq_activate_first_inactive(pwq);
}
}
Expand Down Expand Up @@ -1797,7 +1800,13 @@ static void __queue_work(int cpu, struct workqueue_struct *wq,
pwq->nr_in_flight[pwq->work_color]++;
work_flags = work_color_to_flags(pwq->work_color);

if (likely(pwq->nr_active < pwq->max_active)) {
/*
* Limit the number of concurrently active work items to max_active.
* @work must also queue behind existing inactive work items to maintain
* ordering when max_active changes. See wq_adjust_max_active().
*/
if (list_empty(&pwq->inactive_works) &&
pwq->nr_active < READ_ONCE(pwq->wq->max_active)) {
if (list_empty(&pool->worklist))
pool->watchdog_ts = jiffies;

Expand Down Expand Up @@ -4146,50 +4155,6 @@ static void pwq_release_workfn(struct kthread_work *work)
}
}

/**
* pwq_adjust_max_active - update a pwq's max_active to the current setting
* @pwq: target pool_workqueue
*
* If @pwq isn't freezing, set @pwq->max_active to the associated
* workqueue's saved_max_active and activate inactive work items
* accordingly. If @pwq is freezing, clear @pwq->max_active to zero.
*/
static void pwq_adjust_max_active(struct pool_workqueue *pwq)
{
struct workqueue_struct *wq = pwq->wq;
bool freezable = wq->flags & WQ_FREEZABLE;
unsigned long flags;

/* for @wq->saved_max_active */
lockdep_assert_held(&wq->mutex);

/* fast exit for non-freezable wqs */
if (!freezable && pwq->max_active == wq->saved_max_active)
return;

/* this function can be called during early boot w/ irq disabled */
raw_spin_lock_irqsave(&pwq->pool->lock, flags);

/*
* During [un]freezing, the caller is responsible for ensuring that
* this function is called at least once after @workqueue_freezing
* is updated and visible.
*/
if (!freezable || !workqueue_freezing) {
pwq->max_active = wq->saved_max_active;

while (!list_empty(&pwq->inactive_works) &&
pwq->nr_active < pwq->max_active)
pwq_activate_first_inactive(pwq);

kick_pool(pwq->pool);
} else {
pwq->max_active = 0;
}

raw_spin_unlock_irqrestore(&pwq->pool->lock, flags);
}

/* initialize newly allocated @pwq which is associated with @wq and @pool */
static void init_pwq(struct pool_workqueue *pwq, struct workqueue_struct *wq,
struct worker_pool *pool)
Expand Down Expand Up @@ -4222,9 +4187,6 @@ static void link_pwq(struct pool_workqueue *pwq)
/* set the matching work_color */
pwq->work_color = wq->work_color;

/* sync max_active to the current setting */
pwq_adjust_max_active(pwq);

/* link in @pwq */
list_add_rcu(&pwq->pwqs_node, &wq->pwqs);
}
Expand Down Expand Up @@ -4665,14 +4627,59 @@ static int init_rescuer(struct workqueue_struct *wq)
return 0;
}

/**
* wq_adjust_max_active - update a wq's max_active to the current setting
* @wq: target workqueue
*
* 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.
*/
static void wq_adjust_max_active(struct workqueue_struct *wq)
{
struct pool_workqueue *pwq;

lockdep_assert_held(&wq->mutex);

if ((wq->flags & WQ_FREEZABLE) && workqueue_freezing) {
WRITE_ONCE(wq->max_active, 0);
return;
}

if (wq->max_active == wq->saved_max_active)
return;

/*
* Update @wq->max_active and then kick inactive work items if more
* active work items are allowed. This doesn't break work item ordering
* because new work items are always queued behind existing inactive
* work items if there are any.
*/
WRITE_ONCE(wq->max_active, wq->saved_max_active);

for_each_pwq(pwq, wq) {
unsigned long flags;

/* this function can be called during early boot w/ irq disabled */
raw_spin_lock_irqsave(&pwq->pool->lock, flags);

while (!list_empty(&pwq->inactive_works) &&
pwq->nr_active < wq->max_active)
pwq_activate_first_inactive(pwq);

kick_pool(pwq->pool);

raw_spin_unlock_irqrestore(&pwq->pool->lock, flags);
}
}

__printf(1, 4)
struct workqueue_struct *alloc_workqueue(const char *fmt,
unsigned int flags,
int max_active, ...)
{
va_list args;
struct workqueue_struct *wq;
struct pool_workqueue *pwq;
int len;

/*
Expand Down Expand Up @@ -4711,6 +4718,7 @@ struct workqueue_struct *alloc_workqueue(const char *fmt,

/* init wq */
wq->flags = flags;
wq->max_active = max_active;
wq->saved_max_active = max_active;
mutex_init(&wq->mutex);
atomic_set(&wq->nr_pwqs_to_flush, 0);
Expand Down Expand Up @@ -4739,8 +4747,7 @@ struct workqueue_struct *alloc_workqueue(const char *fmt,
mutex_lock(&wq_pool_mutex);

mutex_lock(&wq->mutex);
for_each_pwq(pwq, wq)
pwq_adjust_max_active(pwq);
wq_adjust_max_active(wq);
mutex_unlock(&wq->mutex);

list_add_tail_rcu(&wq->list, &workqueues);
Expand Down Expand Up @@ -4878,8 +4885,6 @@ EXPORT_SYMBOL_GPL(destroy_workqueue);
*/
void workqueue_set_max_active(struct workqueue_struct *wq, int max_active)
{
struct pool_workqueue *pwq;

/* disallow meddling with max_active for ordered workqueues */
if (WARN_ON(wq->flags & __WQ_ORDERED_EXPLICIT))
return;
Expand All @@ -4890,9 +4895,7 @@ void workqueue_set_max_active(struct workqueue_struct *wq, int max_active)

wq->flags &= ~__WQ_ORDERED;
wq->saved_max_active = max_active;

for_each_pwq(pwq, wq)
pwq_adjust_max_active(pwq);
wq_adjust_max_active(wq);

mutex_unlock(&wq->mutex);
}
Expand Down Expand Up @@ -5139,8 +5142,8 @@ static void show_pwq(struct pool_workqueue *pwq)
pr_info(" pwq %d:", pool->id);
pr_cont_pool_info(pool);

pr_cont(" active=%d/%d refcnt=%d%s\n",
pwq->nr_active, pwq->max_active, pwq->refcnt,
pr_cont(" active=%d refcnt=%d%s\n",
pwq->nr_active, pwq->refcnt,
!list_empty(&pwq->mayday_node) ? " MAYDAY" : "");

hash_for_each(pool->busy_hash, bkt, worker, hentry) {
Expand Down Expand Up @@ -5688,7 +5691,6 @@ EXPORT_SYMBOL_GPL(work_on_cpu_safe_key);
void freeze_workqueues_begin(void)
{
struct workqueue_struct *wq;
struct pool_workqueue *pwq;

mutex_lock(&wq_pool_mutex);

Expand All @@ -5697,8 +5699,7 @@ void freeze_workqueues_begin(void)

list_for_each_entry(wq, &workqueues, list) {
mutex_lock(&wq->mutex);
for_each_pwq(pwq, wq)
pwq_adjust_max_active(pwq);
wq_adjust_max_active(wq);
mutex_unlock(&wq->mutex);
}

Expand Down Expand Up @@ -5763,7 +5764,6 @@ bool freeze_workqueues_busy(void)
void thaw_workqueues(void)
{
struct workqueue_struct *wq;
struct pool_workqueue *pwq;

mutex_lock(&wq_pool_mutex);

Expand All @@ -5775,8 +5775,7 @@ void thaw_workqueues(void)
/* restore max_active and repopulate worklist */
list_for_each_entry(wq, &workqueues, list) {
mutex_lock(&wq->mutex);
for_each_pwq(pwq, wq)
pwq_adjust_max_active(pwq);
wq_adjust_max_active(wq);
mutex_unlock(&wq->mutex);
}

Expand Down

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