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

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Message-ID: <20091012152627.GA27835@Krystal>
Date:	Mon, 12 Oct 2009 11:26:27 -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:
> 
> > > 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.
> 

That's why you should do:

local_irq_save()
preempt_enable_no_resched()
...
local_irq_restore()
preempt_check_resched()

Mathieu



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