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Subject[PATCH 2/2] slub: remove one code path and reduce lock contention in __slab_free()
Date
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>
Cc: Christoph Lameter <cl@linux.com>
Acked-by: Christoph Lameter <cl@linux.com>

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


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