Re: [PATCH] SLUB use cmpxchg_local

Mathieu Desnoyers
2007-08-27T20:39:26Z

* Christoph Lameter (clameter@sgi.com) wrote:
> On Mon, 27 Aug 2007, Mathieu Desnoyers wrote:
>
> > * Christoph Lameter (clameter@sgi.com) wrote:
> > > On Mon, 27 Aug 2007, Peter Zijlstra wrote:
> > >
> > > > So, if the fast path can be done with a preempt off, it might be doable
> > > > to suffer the slow path with a per cpu lock like that.
> > >
> > > Sadly the cmpxchg_local requires local per cpu data access. Isnt there
> > > some way to make this less expensive on RT? Acessing cpu local memory is
> > > really good for performance on NUMA since the data is optimally placed and
> > > one can avoid/reduce locking if the process stays tied to the processor.
> > >
> >
> > On the slow path, in slab_new, we already have to reenable interrupts
> > because we can sleep. If we make sure that whenever we return to an irq
> > disable code path we take the current per-cpu data structure again, can
> > we make the preempt-disable/irq-disabled code paths O(1) ?
>
> Not sure exactly what you are getting at?
> This would mean running __alloc_pages tied to one processor even though
> waiting is possible?
>

Not exactly. What I propose is:

- Running slab_alloc and slab_free fast paths in preempt_disable
context, using cmpxchg_local.
- Running slab_alloc and slab_free slow paths with irqs disabled.
- Running __alloc_pages in preemptible context, not tied to any CPU.

In this scheme, calling __alloc_pages from slab_alloc would reenable
interrupts and potentially migrate us to a different CPU. We would
therefore have to get once again our per-cpu data structure once we get
back into irq disabled code, because we may be running on a different
CPU. This is actually what the __slab_alloc slow path does:

new_slab:
new = get_partial(s, gfpflags, node);
if (new) {
c->page = new;
goto load_freelist;
}

new = new_slab(s, gfpflags, node);

----> within new_slab, we can reenable interrupts for the
__slab_alloc call.

if (new) {
c = get_cpu_slab(s, smp_processor_id());
if (c->page) {
/*
* Someone else populated the cpu_slab while we
* enabled interrupts, or we have gotten scheduled
* on another cpu. The page may not be on the
* requested node even if __GFP_THISNODE was
* specified. So we need to recheck.
*/
if (node_match(c, node)) {
/*
* Current cpuslab is acceptable and we
* want the current one since its cache hot
*/
discard_slab(s, new);
slab_lock(c->page);
goto load_freelist;
}
/* New slab does not fit our expectations */
flush_slab(s, c);
}
slab_lock(new);
SetSlabFrozen(new);
c->page = new;
goto load_freelist;

So the idea would be to split the code in O(1)
preempt_disable/irq_disable sections and to enable interrupt and check
for current per-cpu data structure when re-entering in irq disabled
code.

Mathieu