[PATCH 1/3] workqueue: preserve pwq->stats[] when unbound pwqs are released

[Wilson Felipe Pereira <wfelipe@google.com>]

When an unbound pool_workqueue is replaced (e.g. during CPU hotplug or
apply_workqueue_attrs()), pwq_release_workfn() unlinks and frees the old
pwq along with its pwq->stats[]. This causes cumulative per-workqueue
counters to go backwards.

Add retired_stats[] to struct workqueue_struct and accumulate a released
pwq's stats into wq->retired_stats[] under wq->mutex before freeing it.

Signed-off-by: Wilson Felipe Pereira <wfelipe@google.com>
---
 kernel/workqueue.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index f2e8071a6f4e..88c8cc2b38d8 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -397,6 +397,8 @@ struct workqueue_struct {
 	struct workqueue_attrs	*attrs;	/* PW: workqueue attributes */
 	struct pool_workqueue __rcu *dfl_pwq;   /* PW: only for unbound wqs */
 
+	u64			retired_stats[PWQ_NR_STATS]; /* WQ: stats from released pwqs */
+
 #ifdef CONFIG_SYSFS
 	struct wq_device	*wq_dev;	/* I: for sysfs interface */
 #endif
@@ -5475,8 +5477,12 @@ static void pwq_release_workfn(struct kthread_work *work)
 	 * @wq, and @wq is invalid to access.
 	 */
 	if (!list_empty(&pwq->pwqs_node)) {
+		int i;
+
 		mutex_lock(&wq->mutex);
 		list_del_rcu(&pwq->pwqs_node);
+		for (i = 0; i < PWQ_NR_STATS; i++)
+			wq->retired_stats[i] += pwq->stats[i];
 		is_last = list_empty(&wq->pwqs);
 
 		/*
-- 
2.56.0.218.g1ec904b173-goog