Re: [PATCH 3/3] slub: release a lock if freeing object with a lock isfailed in __slab_free()

[JoonSoo Kim]

2012/7/4 Pekka Enberg <penberg@xxxxxxxxxx>:
> On Fri, Jun 22, 2012 at 9:22 PM, Joonsoo Kim <js1304@xxxxxxxxx> wrote:
>> In some case of __slab_free(), we need a lock for manipulating partial list.
>> If freeing object with a lock is failed, a lock doesn't needed anymore
>> for some reasons.
>>
>> Case 1. prior is NULL, kmem_cache_debug(s) is true
>>
>> In this case, another free is occured before our free is succeed.
>> When slab is full(prior is NULL), only possible operation is slab_free().
>> So in this case, we guess another free is occured.
>> It may make a slab frozen, so lock is not needed anymore.
>>
>> Case 2. inuse is NULL
>>
>> In this case, acquire_slab() is occured before out free is succeed.
>> We have a last object for slab, so other operation for this slab is
>> not possible except acquire_slab().
>> Acquire_slab() makes a slab frozen, so lock is not needed anymore.
>>
>> Above two reason explain why we don't need a lock
>> when freeing object with a lock is failed.
>>
>> So, when cmpxchg_double_slab() is failed, releasing a lock is more suitable.
>> This may reduce lock contention.
>>
>> This also make logic somehow simple that 'was_frozen with a lock' case
>> is never occured. Remove it.
>>
>> Signed-off-by: Joonsoo Kim <js1304@xxxxxxxxx>
>>
>> diff --git a/mm/slub.c b/mm/slub.c
>> index 531d8ed..3e0b9db 100644
>> --- a/mm/slub.c
>> +++ b/mm/slub.c
>> @@ -2438,7 +2438,6 @@ static void __slab_free(struct kmem_cache *s, struct page *page,
>>         void *prior;
>>         void **object = (void *)x;
>>         int was_frozen;
>> -       int inuse;
>>         struct page new;
>>         unsigned long counters;
>>         struct kmem_cache_node *n = NULL;
>> @@ -2450,13 +2449,17 @@ static void __slab_free(struct kmem_cache *s, struct page *page,
>>                 return;
>>
>>         do {
>> +               if (unlikely(n)) {
>> +                       spin_unlock_irqrestore(&n->list_lock, flags);
>> +                       n = NULL;
>> +               }
>>                 prior = page->freelist;
>>                 counters = page->counters;
>>                 set_freepointer(s, object, prior);
>>                 new.counters = counters;
>>                 was_frozen = new.frozen;
>>                 new.inuse--;
>> -               if ((!new.inuse || !prior) && !was_frozen && !n) {
>> +               if ((!new.inuse || !prior) && !was_frozen) {
>>
>>                         if (!kmem_cache_debug(s) && !prior)
>>
>> @@ -2481,7 +2484,6 @@ static void __slab_free(struct kmem_cache *s, struct page *page,
>>
>>                         }
>>                 }
>> -               inuse = new.inuse;
>>
>>         } while (!cmpxchg_double_slab(s, page,
>>                 prior, counters,
>> @@ -2507,25 +2509,17 @@ 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;
>> +
>>         /*
>> -        * was_frozen may have been set after we acquired the list_lock in
>> -        * an earlier loop. So we need to check it here again.
>> +        * Objects left in the slab. If it was not on the partial list before
>> +        * then add it.
>>          */
>> -       if (was_frozen)
>> -               stat(s, FREE_FROZEN);
>> -       else {
>> -               if (unlikely(!inuse && n->nr_partial > s->min_partial))
>> -                        goto slab_empty;
>> -
>> -               /*
>> -                * Objects left in the slab. If it was not on the partial list before
>> -                * then add it.
>> -                */
>> -               if (unlikely(!prior)) {
>> -                       remove_full(s, page);
>> -                       add_partial(n, page, DEACTIVATE_TO_TAIL);
>> -                       stat(s, FREE_ADD_PARTIAL);
>> -               }
>> +       if (kmem_cache_debug(s) && unlikely(!prior)) {
>> +               remove_full(s, page);
>> +               add_partial(n, page, DEACTIVATE_TO_TAIL);
>> +               stat(s, FREE_ADD_PARTIAL);
>>         }
>>         spin_unlock_irqrestore(&n->list_lock, flags);
>>         return;
>
> I'm confused. Does this fix a bug or is it an optimization?

It is for reducing lock contention and code clean-up.

If we aquire a lock and cmpxchg_double failed, we do releasing a lock.
This result in reducing lock contention.
And this remove "was_frozen and having a lock" case, so logic slightly
simpler than before.

Commit message which confuse u means that this patch do not decrease
performance for two reasons.
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