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

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Date:	Mon, 12 Oct 2009 11:38:44 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Christoph Lameter <cl@...ux-foundation.org>
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

* Christoph Lameter (cl@...ux-foundation.org) wrote:
> On Mon, 12 Oct 2009, Mathieu Desnoyers wrote:
> 
> > > If I just enable interrupts there then the preempt check will not be
> > > done and we may miss a scheduling point.
> > >
> >
> > That's why you should do:
> >
> > local_irq_save()
> > preempt_enable_no_resched()
> > ...
> > local_irq_restore()
> > preempt_check_resched()
> 
> What is the difference then to
> 
> local_irq_save()
> 
> ...
> 
> local_irq_enable();
> preempt_enable();
> 
> ?

local_irq_save()
preempt_enable_no_resched()  <- barrier()
...
local_irq_enable()
preempt_check_resched()

vs

local_irq_save()
...
local_irq_enable();
preempt_enable(); <- barrier()

In the first scenario, the compiler barrier is at the beginning of the
slow path function, which should impose less restrictions on the compiler
optimizations.

Mathieu


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