linux-kernel - Re: [RFC][PATCH 2/3] sched, workqueue: Move WQ-sleeper wakeup

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Date:	Wed, 22 Jun 2011 11:37:06 +0200
From:	Tejun Heo <tj@...nel.org>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Jens Axboe <jaxboe@...ionio.com>
Subject: Re: [RFC][PATCH 2/3] sched, workqueue: Move WQ-sleeper wakeup

On Wed, Jun 22, 2011 at 11:27:28AM +0200, Peter Zijlstra wrote:
> On Wed, 2011-06-22 at 11:24 +0200, Tejun Heo wrote:
> > Preemption could still be enabled here, right?  What prevents
> > preemtion kicking after wq_worker_sleeping() and do it again thus
> > breaking nr_running tracking.
> 
> Aren't all PF_WQ_WORKER threads cpu-bound?

Hmmm... I don't see how that would matter.  Please consider the
following scenario.

* A work item calls schedule() for whatever reason.

* schedule() calls sched_submit_work() which in turn calls
  wq_worker_sleeping() which adjusts nr_running.  All this happens
  with preemption enabled.

* sched_submit_work() returns but before schedule() does
  preempt_disable(), an IRQ is delivered.  By the time IRQ return path
  is executed, TIF_NEED_RESCHED is set which in turn calls schedule()
  again and repeats the above two steps for the same worker.

Thanks.

-- 
tejun
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