[PATCH] mm: slub: fix ALLOC_SLOWPATH stat

From: Dave Hansen
Date: Mon Jan 06 2014 - 15:43:06 EST

From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>

There used to be only one path out of __slab_alloc(), and
ALLOC_SLOWPATH got bumped in that exit path. Now there are two,
and a bunch of gotos. ALLOC_SLOWPATH can now get set more than once
during a single call to __slab_alloc() which is pretty bogus.
Here's the sequence:

1. Enter __slab_alloc(), fall through all the way to the
2. hit 'if (!freelist)', and bump DEACTIVATE_BYPASS, jump to
new_slab (goto #1)
3. Hit 'if (c->partial)', bump CPU_PARTIAL_ALLOC, goto redo
(goto #2)
4. Fall through in the same path we did before all the way to
5. bump ALLOC_REFILL stat, then return

Doing this is obviously bogus. It keeps us from being able to
accurately compare ALLOC_SLOWPATH vs. ALLOC_FASTPATH. It also
means that the total number of allocs always exceeds the total
number of frees.

This patch moves stat(s, ALLOC_SLOWPATH) to be called from the
same place that __slab_alloc() is. This makes it much less
likely that ALLOC_SLOWPATH will get botched again in the
spaghetti-code inside __slab_alloc().

Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>

linux.git-davehans/mm/slub.c | 8 +++-----
1 file changed, 3 insertions(+), 5 deletions(-)

diff -puN mm/slub.c~slub-ALLOC_SLOWPATH-stat mm/slub.c
--- linux.git/mm/slub.c~slub-ALLOC_SLOWPATH-stat 2014-01-06 12:39:28.148072544 -0800
+++ linux.git-davehans/mm/slub.c 2014-01-06 12:39:28.155072860 -0800
@@ -2301,8 +2301,6 @@ redo:
if (freelist)
goto load_freelist;

- stat(s, ALLOC_SLOWPATH);
freelist = get_freelist(s, page);

if (!freelist) {
@@ -2409,10 +2407,10 @@ redo:

object = c->freelist;
page = c->page;
- if (unlikely(!object || !node_match(page, node)))
+ if (unlikely(!object || !node_match(page, node))) {
object = __slab_alloc(s, gfpflags, node, addr, c);
- else {
+ stat(s, ALLOC_SLOWPATH);
+ } else {
void *next_object = get_freepointer_safe(s, object);

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