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

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Message-ID: <Pine.LNX.4.64.0708221330530.16037@schroedinger.engr.sgi.com>
Date:	Wed, 22 Aug 2007 13:33:25 -0700 (PDT)
From:	Christoph Lameter <clameter@....com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
cc:	Andi Kleen <andi@...stfloor.org>, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, mingo@...hat.com
Subject: Re: [PATCH] SLUB use cmpxchg_local

On Wed, 22 Aug 2007, Mathieu Desnoyers wrote:

> > Then the thread could be preempted and rescheduled on a different cpu 
> > between put_cpu and local_irq_save() which means that we loose the
> > state information of the kmem_cache_cpu structure.
> > 
> 
> Maybe am I misunderstanding something, but kmem_cache_cpu does not seem
> to be passed to __slab_free() at all, nor any data referenced by it. So
> why do we care about being preempted there ?

Right it is only useful for __slab_alloc. I just changed them both to look 
the same. We could do it that way in __slab_free() to avoid the later 
preempt_check_resched().

> > We could do that but maybe the frequency of these checks would be too 
> > high? When should the resched checks be used?
> 
> Since we are only doing this on the slow path, it does not hurt.
> preempt_check_resched() is embedded in preempt_enable() and has a very
> low impact (simple thread flag check in the standard case).

Ok then lets add it.
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