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

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Date:	Wed, 22 Jun 2011 20:52:47 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Christoph Hellwig <hch@...radead.org>
cc:	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Tejun Heo <tj@...nel.org>, Jens Axboe <axboe@...nel.dk>,
	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, 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.

We don't do that right now as that code conditional on:

    if (prev->state && !(preempt_count() & PREEMPT_ACTIVE)) {

Also blk_flush_plug_list() when called from io_schedule() is
preemptible, so you might be preempted in the middle of the list
operations, so calling into it when preempted would result in an utter
disaster.

Thanks,

	tglx

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