linux-kernel - Re: [this_cpu_xx V5 19/19] SLUB: Experimental new fastpath w/o interrupt disable

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Date:	Wed, 7 Oct 2009 11:02:57 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Christoph Lameter <cl@...ux-foundation.org>
Cc:	Peter Zijlstra <peterz@...radead.org>, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org,
	Pekka Enberg <penberg@...helsinki.fi>,
	Tejun Heo <tj@...nel.org>, Mel Gorman <mel@....ul.ie>,
	mingo@...e.hu
Subject: Re: [this_cpu_xx V5 19/19] SLUB: Experimental new fastpath w/o
	interrupt disable

* Christoph Lameter (cl@...ux-foundation.org) wrote:
> On Wed, 7 Oct 2009, Mathieu Desnoyers wrote:
> 
> > Yes, I understood this is what he was doing, but I wonder about the
> > impact on the scheduler. If we have:
> >
> > * Jiffy 1 -- timer interrupt
> >
> > * preempt disable
> > * Jiffy 2 -- timer interrupt
> >   -> here, the scheduler is disabled, so the timer interrupt is skipped.
> >      The scheduler depends on preempt_check_resched() at preempt_enable()
> >      to execute in a bounded amount of time.
> 
> preempt disable does not disable interrupts. The timer interrupt will
> occur. The scheduler may not reschedule another job on this processor
> when the timer interrupt calls the scheduler_tick. It
> may not do load balancing.

Yes. All you say here is true. I'm concerned about the _impact_ of this
along with the preempt/irqoff dance you propose. Trimming the following
key points from my execution scenario indeed skips the problem altogether.

Usually, when preemption is disabled, the scheduler restrain from
executing. *Now the important point*: the criterion that bounds the
maximum amount of time before the scheduler will re-check for pending
preemption is when preempt_enable() will re-activate preemption.

But because you run preempt_enable with interrupts off, the scheduler
check is not done. And it's not done when interrupts are re-activated
neither.

Please go back to my complete execution scenario, you'll probably see
the light. ;)

Thanks,

Mathieu

> 
> > Also, preempt_enable here should be replaced with
> > preempt_enable_no_resched().
> 
> Used to have that in earlier incarnations but I saw a lot of these being
> removed lately.
> 
> 
> 

-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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