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slub: remove one code path and reduce lock contention in __slab_free() 

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@gmail.com>
Acked-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>
This commit is contained in:
Joonsoo Kim 2012-08-16 00:02:40 +09:00 committed by Pekka Enberg
parent ddffeb8c4d
commit 837d678dc2
1 changed files with 15 additions and 21 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

36
mm/slub.c
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@ -2459,7 +2459,6 @@ 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;
@ -2472,13 +2471,17 @@ 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)
@ -2503,7 +2506,6 @@ static void __slab_free(struct kmem_cache *s, struct page *page,
}
}
inuse = new.inuse;
} while (!cmpxchg_double_slab(s, page,
prior, counters,
@ -2529,25 +2531,17 @@ static void __slab_free(struct kmem_cache *s, struct page *page,
return;
}
/*
* was_frozen may have been set after we acquired the list_lock in
* an earlier loop. So we need to check it here again.
*/
if (was_frozen)
stat(s, FREE_FROZEN);
else {
if (unlikely(!inuse && n->nr_partial > s->min_partial))
goto slab_empty;
if (unlikely(!new.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);
}
/*
* Objects left in the slab. If it was not on the partial list before
* then add it.
*/
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;