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

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Message-ID: <Pine.LNX.4.64.0708221316240.15775@schroedinger.engr.sgi.com>
Date:	Wed, 22 Aug 2007 13:19:16 -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:

> * Christoph Lameter (clameter@....com) wrote:
> >  void *kmem_cache_alloc(struct kmem_cache *s, gfp_t gfpflags)
> > @@ -1577,7 +1590,10 @@ static void __slab_free(struct kmem_cach
> >  {
> >  	void *prior;
> >  	void **object = (void *)x;
> > +	unsigned long flags;
> >  
> > +	local_irq_save(flags);
> > +	put_cpu_no_resched();
> 
> Those two lines may skip a preempt_check.

Yes we cannot execute something else here.

> Could we change them to this instead ?
>   
>   put_cpu();
>   local_irq_save(flags);

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.

> Otherwise, it would be good to call
> 
>   preempt_check_resched();
> 
>   After each local_irq_restore() in this function.

We could do that but maybe the frequency of these checks would be too 
high? When should the resched checks be used?
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