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

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Date:	Wed, 22 Aug 2007 16:29:55 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Christoph Lameter <clameter@....com>
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

* Christoph Lameter (clameter@....com) wrote:
> 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.
> 

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 ?

> > 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?

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).

Mathieu

-- 
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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