linux-kernel - Re: [PATCH 3/5] slab.c: remove branch hint

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Date:	Sun, 29 Nov 2009 12:36:34 +0100
From:	Tim Blechmann <tim@...ngt.org>
To:	Pekka Enberg <penberg@...helsinki.fi>
CC:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	Christoph Lameter <cl@...ux-foundation.org>,
	Nick Piggin <npiggin@...e.de>
Subject: Re: [PATCH 3/5] slab.c: remove branch hint

On 11/24/2009 12:28 PM, Pekka Enberg wrote:
> On Tue, Nov 24, 2009 at 1:20 PM, Ingo Molnar <mingo@...e.hu> wrote:
>> (Pekka Cc:-ed)
>>
>> * Tim Blechmann <tim@...ngt.org> wrote:
>>
>>> branch profiling on my nehalem machine showed 99% incorrect branch hints:
>>>
>>>    28459  7678524  99 __cache_alloc_node             slab.c
>>>   3551
>>>
>>> Signed-off-by: Tim Blechmann <tim@...ngt.org>
>>> ---
>>>  mm/slab.c |    2 +-
>>>  1 files changed, 1 insertions(+), 1 deletions(-)
>>>
>>> diff --git a/mm/slab.c b/mm/slab.c
>>> index f70b326..4125fcd 100644
>>> --- a/mm/slab.c
>>> +++ b/mm/slab.c
>>> @@ -3548,7 +3548,7 @@ __cache_alloc_node(struct kmem_cache *cachep,
>>> gfp_t flags, int nodeid,
>>>       slab_irq_save(save_flags, this_cpu);
>>>       this_node = cpu_to_node(this_cpu);
>>> -     if (unlikely(nodeid == -1))
>>> +     if (nodeid == -1)
>>>               nodeid = this_node;
>>>       if (unlikely(!cachep->nodelists[nodeid])) {
> 
> That sounds odd to me. Can you see where the incorrectly predicted
> calls are coming from? Calling kmem_cache_alloc_node() with node set
> to -1 most of the time could be a real bug somewhere.

when dumping the stack for the incorrectly hinted branches, i get the
attached stack traces...

hth, tim

--- a/mm/slab.c
+++ b/mm/slab.c
@@ -3548,8 +3548,10 @@ __cache_alloc_node(struct kmem_cache *cachep,
gfp_t flags, int nodeid,
        slab_irq_save(save_flags, this_cpu);

        this_node = cpu_to_node(this_cpu);
-       if (nodeid == -1)
+       if (nodeid == -1) {
+               dump_stack();
                nodeid = this_node;
+       }

        if (unlikely(!cachep->nodelists[nodeid])) {
                /* Node not bootstrapped yet */



-- 
tim@...ngt.org
http://tim.klingt.org

It is better to make a piece of music than to perform one, better to
perform one than to listen to one, better to listen to one than to
misuse it as a means of distraction, entertainment, or acquisition of
'culture'.
  John Cage

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