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 08:46:28 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Peter Zijlstra <peterz@...radead.org>
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

* Peter Zijlstra (peterz@...radead.org) wrote:
> On Tue, 2009-10-06 at 22:54 -0400, Mathieu Desnoyers wrote:
> > > +     local_irq_save(flags);
> > > +     preempt_enable();       /* Get rid of count */
> > 
> > Ugh ? Is that legit ?
> 
> Yeah, it reads rather awkward, and the comment doesn't make it any
> better, but what he's doing is:
> 
> slab_alloc()
>   preempt_disable();
>   __slab_alloc()
>     local_irq_save(flags);
>     preempt_enable();
> 

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.
* local_irq_save
* preempt_enable
  -> interrupts are disabled, scheduler execution is skipped.
* local_irq_restore
  -> the interrupt line is low. The scheduler won't be called. There is
     no preempt_check_resched() call.

* Jiffy 3 -- timer interrupt
  -> Scheduler finally gets executed, missing a whole jiffy.

At the very least, I think an explicit preempt_check_resched should be
added after local_irq_restore().

Also, preempt_enable here should be replaced with
preempt_enable_no_resched().

Mathieu

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