linux-kernel - Re: [patch 1/4] sched: Separate the scheduler entry for preemption

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Date:	Wed, 22 Jun 2011 22:15:15 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Jens Axboe <axboe@...nel.dk>
cc:	Christoph Hellwig <hch@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Tejun Heo <tj@...nel.org>, Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [patch 1/4] sched: Separate the scheduler entry for preemption

On Wed, 22 Jun 2011, Jens Axboe wrote:

> On 2011-06-22 20:43, Christoph Hellwig wrote:
> > On Wed, Jun 22, 2011 at 05:52:13PM -0000, Thomas Gleixner wrote:
> >> Block-IO and workqueues call into notifier functions from the
> >> scheduler core code with interrupts and preemption disabled. These
> >> calls should be made before entering the scheduler core.
> >>
> >> To simplify this, separate the scheduler core code into
> >> __schedule(). __schedule() is directly called from the places which
> >> set PREEMPT_ACTIVE and from schedule(). This allows us to add the work
> >> checks into schedule(), so they are only called when a task voluntary
> >> goes to sleep.
> > 
> > I don't think that works.  We'll need to flush the block requests even
> > for an involuntary schedule.
> 
> Yep, doing it just for voluntary schedule() is pointless, since the
> caller should just do the flushing on his own. The whole point of the
> sched hook was to ensure that involuntary schedules flushed it.

I guess we talk about different things here. The involuntary is when
you are preempted, which keeps state unchanged and the current code
already excludes that case.

If you block on a mutex, semaphore, completion or whatever that's a
different thing. That code calls schedule() not __schedule() and that
will flush your stuff as it does now.

Thanks,

	tglx
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