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SubjectRe: [thiscpuops upgrade 10/10] Lockless (and preemptless) fastpaths for slub
* Mathieu Desnoyers (mathieu.desnoyers@efficios.com) wrote:
> * Christoph Lameter (cl@linux.com) wrote:
>
> [...]
>
> > @@ -1737,23 +1770,53 @@ static __always_inline void *slab_alloc(
> > {
> > void **object;
> > struct kmem_cache_cpu *c;
> > - unsigned long flags;
> > + unsigned long tid;
> >
> > if (slab_pre_alloc_hook(s, gfpflags))
> > return NULL;
> >
> > - local_irq_save(flags);
> > +redo:
> > + /*
> > + * Must read kmem_cache cpu data via this cpu ptr. Preemption is
> > + * enabled. We may switch back and forth between cpus while
> > + * reading from one cpu area. That does not matter as long
> > + * as we end up on the original cpu again when doing the cmpxchg.
> > + */
> > c = __this_cpu_ptr(s->cpu_slab);
> > +
> > + /*
> > + * The transaction ids are globally unique per cpu and per operation on
> > + * a per cpu queue. Thus they can be guarantee that the cmpxchg_double
> > + * occurs on the right processor and that there was no operation on the
> > + * linked list in between.
> > + */
>
> There seems to be some voodoo magic I don't understand here. I'm curious to see
> what happens if we have:
>
> CPU A CPU B
> slab_alloc()
> c = __this_cpu_ptr(s->cpu_slab);
> tid = c->tid
> thread migrated to CPU B
>
> slab_alloc()
> c = __this_cpu_ptr(s->cpu_slab);
> tid = c->tid
> ... ...
> irqsafe_cmpxchg_double
> - expect tid, on CPU A, success
> migrate back to CPU A
> irqsafe_cmpxchg_double
> - expect (same) tid, on CPU A, success

Ah! I knew I was missing something: the second cmpxchg will fail because it
expects "tid", but the value is now the "next_tid". So effectively, many
instances of the same transaction can run concurrently, but only one will
succeed.

Sorry for the noise.

Thanks,

Mathieu


>
> So either there is a crucially important point I am missing, or the transaction
> ID does not seem to be truly unique due to migration.
>
> Thanks,
>
> Mathieu
>
>
> > + tid = c->tid;
> > + barrier();
> > +
> > object = c->freelist;
> > - if (unlikely(!object || !node_match(c, node)))
> > + if (unlikely(!object || !node_match(c, c->node)))
> >
> > - object = __slab_alloc(s, gfpflags, node, addr, c);
> > + object = __slab_alloc(s, gfpflags, c->node, addr);
> >
> > else {
> > - c->freelist = get_freepointer(s, object);
> > + /*
> > + * The cmpxchg will only match if there was not additonal
> > + * operation and if we are on the right processor.
> > + */
> > + if (unlikely(!irqsafe_cmpxchg_double(&s->cpu_slab->freelist, object, tid,
> > + get_freepointer(s, object), next_tid(tid)))) {
>
>
> --
> Mathieu Desnoyers
> Operating System Efficiency R&D Consultant
> EfficiOS Inc.
> http://www.efficios.com

--
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com


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