Subject: Re: [this_cpu_xx V5 19/19] SLUB: Experimental new fastpath w/o
 interrupt disable
From: Peter Zijlstra <peterz@infradead.org>
To: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Cc: cl@linux-foundation.org, akpm@linux-foundation.org,
       linux-kernel@vger.kernel.org, Pekka Enberg <penberg@cs.helsinki.fi>,
       Tejun Heo <tj@kernel.org>, Mel Gorman <mel@csn.ul.ie>, mingo@elte.hu
In-Reply-To: <20091007133106.GC29632@Krystal>
References: <20091006233654.815079668@gentwo.org>
	 <20091006233733.153341605@gentwo.org> <20091007025440.GB4664@Krystal>
	 <1254906707.26976.225.camel@twins> <20091007124628.GB27363@Krystal>
	 <1254920460.26976.241.camel@twins>  <20091007133106.GC29632@Krystal>
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Date: Wed, 07 Oct 2009 16:37:13 +0200
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On Wed, 2009-10-07 at 09:31 -0400, Mathieu Desnoyers wrote:
> * Peter Zijlstra (peterz@infradead.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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