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

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Message-ID: <alpine.DEB.1.10.0910071102590.9984@gentwo.org>
Date:	Wed, 7 Oct 2009 11:05:42 -0400 (EDT)
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
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

On Wed, 7 Oct 2009, Mathieu Desnoyers wrote:

> 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.

Which creates additional overhead in the allocator.

> 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.

Ok so we should be moving the preempt_enable after the irq enable. Then we
will call into the scheduler at the end of the slow path. This may add
significantly more overhead that we had before when we simply disabled
and enabled interrupts...


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