On Fri, 2008-05-30 at 11:26 -0700, Christoph Lameter wrote:
> On Fri, 30 May 2008, Peter Zijlstra wrote:
>
> > Please consider adding this get/put interface.
>
> If you could explain why? So far this seems to be based on a
> misunderstanding.
Take for instance kmem_cache_cpu, you currently serialize that by strict
per-cpu-ness and disabling preemption.
The problem is that the preempt-off sections are rather long - so what
we do is add a lock (mutex) and serialize that way - ignoring the cpu
affinity.
so currently:
preempt_disable();
c = get_cpu_slab();
do load of stuff on c;
preempt_enable();
where the preempt-off section is rather long, we'd like to change that
to:
c = get_cpu_slab();
do stuff to c;
put_cpu_slab(c);
so that we can pick between:
!rt
get_cpu_slab(s)
{
preempt_disable();
return THIS_CPU(s->cpu_slab);
}
put_cpu_slab(c)
{
preempt_enable();
}
-rt:
get_cpu_slab(s)
{
c = THIS_CPU(s->cpu_slab);
spin_lock(&c->lock); // <-- really a PI-mutex
}
put_cpu_slab(c)
{
spin_unlock(&c->lock);
}
Also, it explicitly ties the preempt-off section to the data used in
case of !rt, which in turn allows for the direct conversion to the
locked version.
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