linux-kernel - Re: [PATCH 05/10] block: remove per-queue plugging

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Date:	Wed, 13 Apr 2011 13:12:02 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	NeilBrown <neilb@...e.de>, Dave Chinner <david@...morbit.com>,
	Jens Axboe <jaxboe@...ionio.com>,
	"hch@...radead.org" <hch@...radead.org>,
	Mike Snitzer <snitzer@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"dm-devel@...hat.com" <dm-devel@...hat.com>,
	"linux-raid@...r.kernel.org" <linux-raid@...r.kernel.org>
Subject: Re: [PATCH 05/10] block: remove per-queue plugging

On Tue, 2011-04-12 at 19:23 -0700, Linus Torvalds wrote:
>  kernel/sched.c |   20 ++++++++++----------
>  1 files changed, 10 insertions(+), 10 deletions(-)
> 
> diff --git a/kernel/sched.c b/kernel/sched.c
> index 48013633d792..a187c3fe027b 100644
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -4111,20 +4111,20 @@ need_resched:
>                                         try_to_wake_up_local(to_wakeup);
>                         }
>                         deactivate_task(rq, prev, DEQUEUE_SLEEP);
> +
> +                       /*
> +                        * If we are going to sleep and we have plugged IO queued, make
> +                        * sure to submit it to avoid deadlocks.
> +                        */
> +                       if (blk_needs_flush_plug(prev)) {
> +                               raw_spin_unlock(&rq->lock);
> +                               blk_flush_plug(prev);
> +                               raw_spin_lock(&rq->lock);
> +                       }
>                 }
>                 switch_count = &prev->nvcsw;
>         }
>  
> -       /*
> -        * If we are going to sleep and we have plugged IO queued, make
> -        * sure to submit it to avoid deadlocks.
> -        */
> -       if (prev->state != TASK_RUNNING && blk_needs_flush_plug(prev)) {
> -               raw_spin_unlock(&rq->lock);
> -               blk_flush_plug(prev);
> -               raw_spin_lock(&rq->lock);
> -       }
> -
>         pre_schedule(rq, prev);
>  
>         if (unlikely(!rq->nr_running)) 

Right, that cures the preemption problem. The reason I suggested placing
it where it was is that I'd like to keep all things that release
rq->lock in the middle of schedule() in one place, but I guess we can
cure that with some extra comments.



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