Re: [PATCH] SLUB use cmpxchg_local

Mathieu Desnoyers
2007-08-21T23:50:34Z

* Mathieu Desnoyers (mathieu.desnoyers@polymtl.ca) wrote:
> * Christoph Lameter (clameter@sgi.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.