Re: [PATCH] mm: slub: fix ALLOC_SLOWPATH stat

[Wanpeng Li <liwanp@linux.vnet.ibm.com>]

On Mon, Jan 06, 2014 at 12:43:00PM -0800, Dave Hansen wrote:
>
>From: Dave Hansen <dave.hansen@linux.intel.com>
>
>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
>   stat(s, ALLOC_SLOWPATH);
>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
>   stat(s, ALLOC_SLOWPATH)
>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@linux.intel.com>

Reviewed-by: Wanpeng Li <liwanp@linux.vnet.ibm.com>

>---
>
> 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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