[PATCH 6.8 003/715] workqueue: Move pwq->max_active to wq->max_active

Sasha Levin
2024-03-24T22:35:01Z

From: Tejun Heo <tj@kernel.org>

[ Upstream commit a045a272d887575da17ad86d6573e82871b50c27 ]

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>
Stable-dep-of: 5797b1c18919 ("workqueue: Implement system-wide nr_active enforcement for unbound workqueues")
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
kernel/workqueue.c | 133 ++++++++++++++++++++++-----------------------
1 file changed, 66 insertions(+), 67 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 8a06fddb23e66..8ee754dd319da 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -143,6 +143,9 @@ enum {
*
* 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.
@@ -250,7 +253,6 @@ 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 */
@@ -298,7 +300,8 @@ 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 */
@@ -1492,7 +1495,7 @@ 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);
}
}
@@ -1793,7 +1796,13 @@ 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;

@@ -4142,50 +4151,6 @@ 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)
@@ -4218,9 +4183,6 @@ 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);
}
@@ -4658,6 +4620,52 @@ 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,
@@ -4665,7 +4673,6 @@ struct workqueue_struct *alloc_workqueue(const char *fmt,
{
va_list args;
struct workqueue_struct *wq;
- struct pool_workqueue *pwq;
int len;

/*
@@ -4704,6 +4711,7 @@ 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);
@@ -4732,8 +4740,7 @@ 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);
@@ -4871,8 +4878,6 @@ 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;
@@ -4883,9 +4888,7 @@ 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);
}
@@ -5132,8 +5135,8 @@ 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) {
@@ -5681,7 +5684,6 @@ 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);

@@ -5690,8 +5692,7 @@ 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);
}

@@ -5756,7 +5757,6 @@ bool freeze_workqueues_busy(void)
void thaw_workqueues(void)
{
struct workqueue_struct *wq;
- struct pool_workqueue *pwq;

mutex_lock(&wq_pool_mutex);

@@ -5768,8 +5768,7 @@ 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);
}

--
2.43.0