linux-kernel - [PATCH] kernel: workqueue: clarify wq_worker_last

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Message-Id: <20190206150528.31198-1-hannes@cmpxchg.org>
Date:   Wed,  6 Feb 2019 16:05:28 +0100
From:   Johannes Weiner <hannes@...xchg.org>
To:     Tejun Heo <tj@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, kernel-team@...com
Subject: [PATCH] kernel: workqueue: clarify wq_worker_last_func() caller requirements

This function can only be called safely from very specific scheduler
contexts. Document those.

Suggested-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Johannes Weiner <hannes@...xchg.org>
---
 kernel/workqueue.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

Andrew suggested including the explanations that came up during the
code review in the function doc. As the function has since been
merged, sending as follow-up for 5.1. Thanks!

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index fc5d23d752a5..23a67b9430da 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -918,6 +918,16 @@ struct task_struct *wq_worker_sleeping(struct task_struct *task)
  * CONTEXT:
  * spin_lock_irq(rq->lock)
  *
+ * 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.
+ *
  * Return:
  * The last work function %current executed as a worker, NULL if it
  * hasn't executed any work yet.
-- 
2.20.1