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 09:31:06 -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 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.

> I tried to find the actual code, but frigging paravirt crap obfuscated
> the code enough that I actually gave up.

You're probably looking for:

arch/x86/include/asm/irqflags.h:

static inline void native_irq_enable(void)
{
        asm volatile("sti": : :"memory");
}

and

static inline void native_restore_fl(unsigned long flags)
{
        asm volatile("push %0 ; popf"
                     : /* no output */
                     :"g" (flags)
                     :"memory", "cc");
}

static inline void raw_local_irq_restore(unsigned long flags)
{
        native_restore_fl(flags);
}

Which are as simple as it gets.

Thanks,

Mathieu

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