linux-kernel - Re: [PATCH] SLUB use cmpxchg

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Message-ID: <20070821235023.GF29691@Krystal>
Date:	Tue, 21 Aug 2007 19:50:23 -0400
From:	Mathieu Desnoyers <compudj@...stal.dyndns.org>
To:	Christoph Lameter <clameter@....com>
Cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	mingo@...hat.com
Subject: Re: [PATCH] SLUB use cmpxchg_local

* Mathieu Desnoyers (mathieu.desnoyers@...ymtl.ca) wrote:
> * Christoph Lameter (clameter@....com) wrote:
> > On Tue, 21 Aug 2007, Mathieu Desnoyers wrote:
> > 
> > > - Changed smp_rmb() for barrier(). We are not interested in read order
> > >   across cpus, what we want is to be ordered wrt local interrupts only.
> > >   barrier() is much cheaper than a rmb().
> > 
> > But this means a preempt disable is required. RT users do not want that.
> > Without preemption the processor can be moved after c has been determined.
> > That is why the smp_rmb() is there.
> 
> preemption is required if we want to use cmpxchg_local anyway.
> 
> We may have to find a way to use preemption while being able to give an
> upper bound on the preempt disabled execution time. I think I got a way
> to do this yesterday.. I'll dig in my patches.
> 

Yeah, I remember having done so : moving the preempt disable nearer to
the cmpxchg, checking if the cpuid has changed between the
raw_smp_processor_id() read and the preempt_disable done later, redo if
it is different. It makes the slow path faster, but makes the fast path
more complex, therefore I finally dropped the patch. And we talk about
~10 cycles for the slow path here, I doubt it's worth the complexity
added to the fast path.

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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