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

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Date:	Thu, 08 Oct 2009 14:53:55 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	Christoph Lameter <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 Thu, 2009-10-08 at 08:44 -0400, Mathieu Desnoyers wrote:
> Even if only done with interrupt off, and check resched is called after
> each irq enable following this critical section ? I'd like to understand
> the reason behind your rejection for this specific case.

No, the thing you proposed:

> preempt disable()
> fast path attempt
>   if (fast path already taken) {
>     local_irq_save();
>     preempt_enable_no_resched();
>     slow path {
>       if (!flags & GFP_ATOMIC) {
>         local_irq_enable();
>         preempt_check_resched();
>         ...
>         local_irq_disable();
>       }
>     }
>     local_irq_restore();
>     preempt_check_resched();
>     return;
>   }
> preempt_enable()

Seems ok.

I just don't get why Christoph is getting all upset about the
need_resched() check in preempt_enable(), its still cheaper than poking
at the interrupt flags.
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