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

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Date:	Wed, 22 Jun 2011 15:47:29 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Tejun Heo <tj@...nel.org>
cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>, Jens Axboe <jaxboe@...ionio.com>
Subject: Re: [RFC][PATCH 2/3] sched, workqueue: Move WQ-sleeper wakeup

On Wed, 22 Jun 2011, Tejun Heo wrote:
> On Wed, Jun 22, 2011 at 01:34:46AM +0200, Peter Zijlstra 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.

That wq code should serialize itself and not magically abuse rq->lock
for this.
 
> >  		 * If we are going to sleep and we have plugged IO
> >  		 * queued, make sure to submit it to avoid deadlocks.
> >  		 */
> > @@ -4256,19 +4235,6 @@ asmlinkage void __sched schedule(void)
> >  		} else {
> >  			deactivate_task(rq, prev, DEQUEUE_SLEEP);
> >  			prev->on_rq = 0;
> > -
> > -			/*
> > -			 * If a worker went to sleep, notify and ask workqueue
> > -			 * whether it wants to wake up a task to maintain
> > -			 * concurrency.
> > -			 */
> > -			if (prev->flags & PF_WQ_WORKER) {
> > -				struct task_struct *to_wakeup;
> > -
> > -				to_wakeup = wq_worker_sleeping(prev, cpu);
> > -				if (to_wakeup)
> > -					try_to_wake_up_local(to_wakeup);
> > -			}
> 
> Similarly, the if the 'if {}' part of the above if/else is taken, the
> task never goes to sleep and nr_running will again be broken.

The accounting can be done at schedule entry and exit and not
somewhere magic in the wakeup code. It does really not matter, whether
nr_running is updated on wakeup or when the worker gets on the cpu.

Thanks,

	tglx
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