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 10:42:11 -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:

> 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.

> 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.



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