linux-kernel - Re: [PATCHv4 17/17] writeback: lessen sync

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Date:	Mon, 31 May 2010 16:03:35 +0300
From:	Artem Bityutskiy <dedekind1@...il.com>
To:	Nick Piggin <npiggin@...e.de>
Cc:	Al Viro <viro@...IV.linux.org.uk>,
	LKML <linux-kernel@...r.kernel.org>,
	Jens Axboe <jens.axboe@...cle.com>,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCHv4 17/17] writeback: lessen sync_supers wakeup count

On Mon, 2010-05-31 at 22:47 +1000, Nick Piggin wrote:
> > set_current_state(TASK_INTERRUPTIBLE);
> > /* 
> >  * XXX: what if we are preempted here. No timer is armed. Our state is
> >  * TASK_INTERRUPTIBLE, supers_dirty is 1, so no one will ever wake us
> >  * up. Thus, we'll sleep forever.
> >  */
> > if (supers_dirty)
> > 	bdi_arm_supers_timer();
> > schedule();
> > 
> > Not sure, but I did quick search and it looks like in preemptive kernel,
> > an interrupt may happen in the XXX place above, then it will call
> > 'preempt_schedule_irq()', which sill call 'schedule()'.
> 
> Yes, preempt does not participate in tsak sleeping exactly for reasons
> such as this.
> 
> From kernel/sched.c:schedule()
> 
>         if (prev->state && !(preempt_count() & PREEMPT_ACTIVE)) {
>                 if (unlikely(signal_pending_state(prev->state, prev)))
>                         prev->state = TASK_RUNNING;
>                 else
>                         deactivate_task(rq, prev, DEQUEUE_SLEEP);
>                 switch_count = &prev->nvcsw;
>         }
> 
> If the task is not running, then is only removed from the runqueue
> (or reset to running in case of pending signal) IFF it has not been
> scheduled from an involuntary kernel preemption.
> 
> So in the XXX region, the task will actually be allowed to run again
> until it calls schedule().

Clear now, thanks a lot again!

-- 
Best Regards,
Artem Bityutskiy (Артём Битюцкий)

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