mm: optimize put_mems_allowed() usage

Since put_mems_allowed() is strictly optional, its a seqcount retry, we
don't need to evaluate the function if the allocation was in fact
successful, saving a smp_rmb some loads and comparisons on some relative
fast-paths.

Since the naming, get/put_mems_allowed() does suggest a mandatory
pairing, rename the interface, as suggested by Mel, to resemble the
seqcount interface.

This gives us: read_mems_allowed_begin() and read_mems_allowed_retry(),
where it is important to note that the return value of the latter call
is inverted from its previous incarnation.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Mel Gorman <mgorman@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 2 insertions, 2 deletions

diff --git a/mm/slab.c b/mm/slab.c
index b264214c77ea..9153c802e2fe 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -3073,7 +3073,7 @@ static void *fallback_alloc(struct kmem_cache *cache, gfp_t flags)
         local_flags = flags & (GFP_CONSTRAINT_MASK|GFP_RECLAIM_MASK);
 
 retry_cpuset:
-        cpuset_mems_cookie = get_mems_allowed();
+        cpuset_mems_cookie = read_mems_allowed_begin();
         zonelist = node_zonelist(slab_node(), flags);
 
 retry:
@@ -3131,7 +3131,7 @@ retry:
                 }
         }
 
-        if (unlikely(!put_mems_allowed(cpuset_mems_cookie) && !obj))
+        if (unlikely(!obj && read_mems_allowed_retry(cpuset_mems_cookie)))
                 goto retry_cpuset;
         return obj;
 }