Re: [PATCH 2/2] slub: remove one code path and reduce lock contentionin __slab_free()

[JoonSoo Kim]

2012/8/25 JoonSoo Kim <js1304@xxxxxxxxx>:
> 2012/8/16 Joonsoo Kim <js1304@xxxxxxxxx>:
>> When we try to free object, there is some of case that we need
>> to take a node lock. This is the necessary step for preventing a race.
>> After taking a lock, then we try to cmpxchg_double_slab().
>> But, there is a possible scenario that cmpxchg_double_slab() is failed
>> with taking a lock. Following example explains it.
>>
>> CPU A               CPU B
>> need lock
>> ...                 need lock
>> ...                 lock!!
>> lock..but spin      free success
>> spin...             unlock
>> lock!!
>> free fail
>>
>> In this case, retry with taking a lock is occured in CPU A.
>> I think that in this case for CPU A,
>> "release a lock first, and re-take a lock if necessary" is preferable way.
>>
>> There are two reasons for this.
>>
>> First, this makes __slab_free()'s logic somehow simple.
>> With this patch, 'was_frozen = 1' is "always" handled without taking a lock.
>> So we can remove one code path.
>>
>> Second, it may reduce lock contention.
>> When we do retrying, status of slab is already changed,
>> so we don't need a lock anymore in almost every case.
>> "release a lock first, and re-take a lock if necessary" policy is
>> helpful to this.
>>
>> Signed-off-by: Joonsoo Kim <js1304@xxxxxxxxx>
>> Cc: Christoph Lameter <cl@xxxxxxxxx>
>> Acked-by: Christoph Lameter <cl@xxxxxxxxx>
>>
>> diff --git a/mm/slub.c b/mm/slub.c
>> index ca778e5..efce427 100644
>> --- a/mm/slub.c
>> +++ b/mm/slub.c
>> @@ -2421,7 +2421,6 @@ 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;
>> @@ -2433,13 +2432,17 @@ 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)
>>
>> @@ -2464,7 +2467,6 @@ static void __slab_free(struct kmem_cache *s, struct page *page,
>>
>>                         }
>>                 }
>> -               inuse = new.inuse;
>>
>>         } while (!cmpxchg_double_slab(s, page,
>>                 prior, counters,
>> @@ -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;
>> +
>>         /*
>> -        * 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;
>> --
>> 1.7.9.5
>>
>
> Hello, Pekka.
> Could you review this patch and comment it, please?

Hello, Pekka.
Resend for ping.
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