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SubjectRe: [thiscpuops upgrade 10/10] Lockless (and preemptless) fastpaths for slub
* 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

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


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