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SubjectRe: [PATCH 3/3] slub: release a lock if freeing object with a lock is failed in __slab_free()
From
2012/7/4 Pekka Enberg <penberg@kernel.org>:
> On Fri, Jun 22, 2012 at 9:22 PM, Joonsoo Kim <js1304@gmail.com> 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@gmail.com>
>>
>> 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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