Re: [PATCH] slub: remove one code path and reduce lock contention in __slab_free()

[Christoph Lameter <cl@linux.com>]

On Sat, 28 Jul 2012, JoonSoo Kim wrote:

> 2012/7/28 Christoph Lameter <cl@linux.com>:
> > On Sat, 28 Jul 2012, Joonsoo Kim wrote:
> >
> >> Subject and commit log are changed from v1.
> >
> > That looks a bit better. But the changelog could use more cleanup and
> > clearer expression.
> >
> >> @@ -2490,25 +2492,17 @@ static void __slab_free(struct kmem_cache *s, struct page *page,
> >>                  return;
> >>          }
> >>
> >> +     if (unlikely(!new.inuse && n->nr_partial > s->min_partial))
> >> +             goto slab_empty;
> >> +
> >
> > So we can never encounter a empty slab that was frozen before? Really?
>
> In my suggestion,  'was_frozen = 1' is "always" handled without taking a lock.

Yepo that is true with this patch.

> Then, never hit following code.
> +     if (unlikely(!new.inuse && n->nr_partial > s->min_partial))
> +             goto slab_empty;
> +


Correct.

> Instead, hit following code.
>         if (likely(!n)) {
>
>                 /*
>                  * If we just froze the page then put it onto the
>                  * per cpu partial list.
>                  */
>                 if (new.frozen && !was_frozen) {
>                         put_cpu_partial(s, page, 1);
>                         stat(s, CPU_PARTIAL_FREE);
>                 }
>                 /*
>                  * The list lock was not taken therefore no list
>                  * activity can be necessary.
>                  */
>                 if (was_frozen)
>                         stat(s, FREE_FROZEN);
>                 return;
>         }
>
> So, even if we encounter a empty slab that was frozen before, we just
> do "stat(s, FREE_FROZEN)".
> Please let me know my answer is sufficient.

Yes.

Acked-by: Christoph Lameter <cl@linux.com>

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