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

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Message-ID: <20091007151940.GA10052@Krystal>
Date:	Wed, 7 Oct 2009 11:19:40 -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:
> 
> > 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.
> 

Which we like to keep as low as possible, I agree.

> > 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...
> 

You are already calling the scheduler when ending the _fast_ path. I
don't see the problem with calling it when you end the slow path
execution.

Mathieu

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