From: Linus Torvalds torvalds@linux-foundation.org
commit 9b1ea29bc0d7b94d420f96a0f4121403efc3dd85 upstream.
This reverts commit 8ff60eb052eeba95cfb3efe16b08c9199f8121cf.
The kernel test robot reports a huge performance regression due to the commit, and the reason seems fairly straightforward: when there is contention on the page list (which is what causes acquire_slab() to fail), we do _not_ want to just loop and try again, because that will transfer the contention to the 'n->list_lock' spinlock we hold, and just make things even worse.
This is admittedly likely a problem only on big machines - the kernel test robot report comes from a 96-thread dual socket Intel Xeon Gold 6252 setup, but the regression there really is quite noticeable:
-47.9% regression of stress-ng.rawpkt.ops_per_sec
and the commit that was marked as being fixed (7ced37197196: "slub: Acquire_slab() avoid loop") actually did the loop exit early very intentionally (the hint being that "avoid loop" part of that commit message), exactly to avoid this issue.
The correct thing to do may be to pick some kind of reasonable middle ground: instead of breaking out of the loop on the very first sign of contention, or trying over and over and over again, the right thing may be to re-try _once_, and then give up on the second failure (or pick your favorite value for "once"..).
Reported-by: kernel test robot oliver.sang@intel.com Link: https://lore.kernel.org/lkml/20210301080404.GF12822@xsang-OptiPlex-9020/ Cc: Jann Horn jannh@google.com Cc: David Rientjes rientjes@google.com Cc: Joonsoo Kim iamjoonsoo.kim@lge.com Acked-by: Christoph Lameter cl@linux.com Signed-off-by: Linus Torvalds torvalds@linux-foundation.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- mm/slub.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/mm/slub.c b/mm/slub.c index f5e94499b736f..af7343a744091 100644 --- a/mm/slub.c +++ b/mm/slub.c @@ -1819,7 +1819,7 @@ static void *get_partial_node(struct kmem_cache *s, struct kmem_cache_node *n,
t = acquire_slab(s, n, page, object == NULL, &objects); if (!t) - continue; /* cmpxchg raced */ + break;
available += objects; if (!object) {