Re: [PATCH] SLUB use cmpxchg_local

Mathieu Desnoyers
2007-08-22T20:35:22Z

* Christoph Lameter (clameter@sgi.com) wrote:
> On Wed, 22 Aug 2007, Mathieu Desnoyers wrote:
>
> > * Christoph Lameter (clameter@sgi.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