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 10:52:29 -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 Mon, 12 Oct 2009, Mathieu Desnoyers wrote:

> > In the middle of the function we have determine that we have to go to the
> > page allocator to get more memory. There is not much the compiler can do
> > to speed that up.
>
> Indeed, the compiler cannot do much about it. However, the programer
> (you) can move the preempt_enable_no_resched() part of the
> preempt_enable() to the beginning of the function.

Ok but then we have the issue that the later irq enable in the
slowpath will not check for preemption.

> > > I still recommend the preempt_enable_no_resched() at the beginning of
> > > __slab_alloc(), and simply putting a check_resched() here (which saves
> > > us the odd compiler barrier in the middle of function).
> >
> > Then preemption would be unnecessarily disabled for the page allocator
> > call?
>
> No ?
> preempt_enable_no_resched() enables preemption.

If I just enable interrupts there then the preempt check will not be
done and we may miss a scheduling point.

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