From: Abel Wu <wuyun.wu@xxxxxxxxxx> Subject: 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 Link: http://lkml.kernel.org/r/20200813101812.1617-1-wuyun.wu@xxxxxxxxxx Signed-off-by: Abel Wu <wuyun.wu@xxxxxxxxxx> Acked-by: Christoph Lameter <cl@xxxxxxxxx> Cc: Pekka Enberg <penberg@xxxxxxxxxx> Cc: David Rientjes <rientjes@xxxxxxxxxx> Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx> Cc: Hewenliang <hewenliang4@xxxxxxxxxx> Cc: Hu Shiyuan <hushiyuan@xxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- mm/slub.c | 23 ++++++++++++----------- 1 file changed, 12 insertions(+), 11 deletions(-) --- a/mm/slub.c~mm-slub-branch-optimization-in-free-slowpath +++ a/mm/slub.c @@ -3019,20 +3019,21 @@ static void __slab_free(struct kmem_cach 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; } _
