From 9b1ea29bc0d7b94d420f96a0f4121403efc3dd85 Mon Sep 17 00:00:00 2001 From: Linus Torvalds Date: Wed, 10 Mar 2021 10:18:04 -0800 Subject: [PATCH] Revert "mm, slub: consider rest of partial list if acquire_slab() fails" 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 Link: https://lore.kernel.org/lkml/20210301080404.GF12822@xsang-OptiPlex-9020/ Cc: Jann Horn Cc: David Rientjes Cc: Joonsoo Kim Acked-by: Christoph Lameter Signed-off-by: Linus Torvalds --- mm/slub.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/mm/slub.c b/mm/slub.c index e26c274b4657..3021ce9bf1b3 100644 --- a/mm/slub.c +++ b/mm/slub.c @@ -1993,7 +1993,7 @@ 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) {