From: Tejun Heo <tj@kernel.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: LKML <linux-kernel@vger.kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
Jens Axboe <axboe@kernel.dk>, Ingo Molnar <mingo@elte.hu>,
Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [patch 4/4] sched: Distangle worker accounting from rq->lock
Date: Thu, 23 Jun 2011 17:07:20 +0200 [thread overview]
Message-ID: <20110623150720.GR30101@htj.dyndns.org> (raw)
In-Reply-To: <20110622174919.135236139@linutronix.de>
Hello, again.
So, let's get it correct first.
On Wed, Jun 22, 2011 at 05:52:15PM -0000, Thomas Gleixner wrote:
> The worker accounting for cpu bound workers is plugged into the core
> scheduler code and the wakeup code. This is not a hard requirement and
> can be avoided by keeping track of the state in the workqueue code
> itself.
>
> Keep track of the sleeping state in the worker itself and call the
> notifier before entering the core scheduler. There might be false
> positives when the task is woken between that call and actually
> scheduling, but that's not really different from scheduling and being
> woken immediately after switching away. There is also no harm from
> updating nr_running when the task returns from scheduling instead of
> accounting it in the wakeup code.
I think false positives on schedule() should be safe. As said
earlier, the gap between ttwu and actually running is a bit worrisome
but it might be nothing, but please at least describe the behavior
change.
> Index: linux-2.6/kernel/workqueue.c
> ===================================================================
> --- linux-2.6.orig/kernel/workqueue.c
> +++ linux-2.6/kernel/workqueue.c
> @@ -137,6 +137,7 @@ struct worker {
> unsigned int flags; /* X: flags */
> int id; /* I: worker id */
> struct work_struct rebind_work; /* L: rebind worker to cpu */
> + int sleeping; /* None */
bool?
> -struct task_struct *wq_worker_sleeping(struct task_struct *task,
> - unsigned int cpu)
> +void wq_worker_sleeping(struct task_struct *task)
> {
> - struct worker *worker = kthread_data(task), *to_wakeup = NULL;
> - struct global_cwq *gcwq = get_gcwq(cpu);
> - atomic_t *nr_running = get_gcwq_nr_running(cpu);
> + struct worker *worker = kthread_data(task);
> + struct global_cwq *gcwq;
> + int cpu;
>
> if (worker->flags & WORKER_NOT_RUNNING)
> - return NULL;
> + return;
This doesn't look safe. It can race with trustee_thread() setting
WORKER_ROGUE. Let's just grab gcwq->lock on entry to
wq_worker_sleeping() for now; then, the schedule() trickery in
trustee_thread() can go away too. This also means we can remove the
weird sync rules from ->flags and ->idle_list and just use simple
gcwq->lock for those, which is pretty nice.
> - /* this can only happen on the local cpu */
> - BUG_ON(cpu != raw_smp_processor_id());
> + if (WARN_ON_ONCE(worker->sleeping))
> + return;
Re-entrance is prevented by the scheduler hook being called only for
non-premption schedule(). Maybe it's better to explain that in the
function comment?
Hmmm... Also, I think worker->sleeping should be cleared by
trustee_thread() when WORKER_ROGUE is set for the worker; otherwise,
it can get out of sync and the above WARN_ON_ONCE() will trigger.
Thanks.
--
tejun
next prev parent reply other threads:[~2011-06-23 15:07 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-06-22 17:52 [patch 0/4] sched: Move work out of the scheduler core Thomas Gleixner
2011-06-22 17:52 ` [patch 1/4] sched: Separate the scheduler entry for preemption Thomas Gleixner
2011-06-22 18:43 ` Christoph Hellwig
2011-06-22 18:52 ` Thomas Gleixner
2011-06-22 19:42 ` Jens Axboe
2011-06-22 20:15 ` Thomas Gleixner
2011-06-23 11:41 ` Jens Axboe
2011-08-29 14:55 ` [tip:sched/urgent] " tip-bot for Thomas Gleixner
2011-06-22 17:52 ` [patch 3/4] block: Shorten interrupt disabled regions Thomas Gleixner
2011-06-22 17:52 ` [patch 2/4] sched: Move blk_schedule_flush_plug() out of __schedule() Thomas Gleixner
2011-06-22 17:52 ` [patch 4/4] sched: Distangle worker accounting from rq->lock Thomas Gleixner
2011-06-22 19:30 ` Thomas Gleixner
2011-06-23 8:37 ` Tejun Heo
2011-06-23 9:58 ` Thomas Gleixner
2011-06-23 10:15 ` Tejun Heo
2011-06-23 10:44 ` Ingo Molnar
2011-06-23 11:35 ` Tejun Heo
2011-06-23 12:51 ` Ingo Molnar
2011-06-24 9:01 ` Thomas Gleixner
2011-06-26 10:19 ` Tejun Heo
2011-06-23 15:07 ` Tejun Heo [this message]
2013-04-30 13:37 ` Steven Rostedt
2013-04-30 22:47 ` Steven Rostedt
2013-05-03 0:12 ` Tejun Heo
2013-05-03 0:57 ` Steven Rostedt
2013-07-24 10:04 ` Thomas Gleixner
2013-08-06 19:33 ` Steven Rostedt
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