mm/slub.c: branch optimization in free slowpath

The two conditions are mutually exclusive and gcc compiler will optimise
this into if-else-like pattern.  Given that the majority of free_slowpath
is free_frozen, let's provide some hint to the compilers.

Tests (perf bench sched messaging -g 20 -l 400000, executed 10x
after reboot) are done and the summarized result:

	un-patched	patched
max.	192.316		189.851
min.	187.267		186.252
avg.	189.154		188.086
stdev.	1.37		0.99

Signed-off-by: Abel Wu <wuyun.wu@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Hewenliang <hewenliang4@huawei.com>
Cc: Hu Shiyuan <hushiyuan@huawei.com>
Link: http://lkml.kernel.org/r/20200813101812.1617-1-wuyun.wu@huawei.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/slub.c b/mm/slub.c
index 6d3574013b2f8cf745e9f2ffeab9b819e8b27546..da6438bd820254ce6c42d344f14aa1be048e0212 100644 (file)
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -3019,20 +3019,21 @@ static void __slab_free(struct kmem_cache *s, struct page *page,
 
        if (likely(!n)) {
 
-               /*
-                * If we just froze the page then put it onto the
-                * per cpu partial list.
-                */
-               if (new.frozen && !was_frozen) {
+               if (likely(was_frozen)) {
+                       /*
+                        * The list lock was not taken therefore no list
+                        * activity can be necessary.
+                        */
+                       stat(s, FREE_FROZEN);
+               } else if (new.frozen) {
+                       /*
+                        * If we just froze the page then put it onto the
+                        * per cpu partial list.
+                        */
                        put_cpu_partial(s, page, 1);
                        stat(s, CPU_PARTIAL_FREE);
                }
-               /*
-                * The list lock was not taken therefore no list
-                * activity can be necessary.
-                */
-               if (was_frozen)
-                       stat(s, FREE_FROZEN);
+
                return;
        }