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

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Date:	Wed, 07 Oct 2009 16:37:13 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	cl@...ux-foundation.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, 2009-10-07 at 09:31 -0400, Mathieu Desnoyers wrote:
> * Peter Zijlstra (peterz@...radead.org) wrote:
> > On Wed, 2009-10-07 at 08:46 -0400, Mathieu Desnoyers wrote:
> > > * local_irq_restore
> > >   -> the interrupt line is low. The scheduler won't be called. There is
> > >      no preempt_check_resched() call.
> > 
> > That would be an issue with all irq disable sections, so I don't think
> > this is actually true.
> > 
> 
> AFAIK, irq disable sections rely on the fact that if you get a timer
> interrupt during this section, the timer interrupt line stays triggered
> for the duration of the irqoff section. Therefore, when interrupts are
> re-enabled, the interrupt kicks in, so does the scheduler.
> 
> This is not the case with the preempt/irqoff dance proposed by
> Christoph.

Ah, you're quite right indeed.

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