tim at klingt
Nov 29, 2009, 3:36 AM
Post #8 of 12
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Re: [PATCH 3/5] slab.c: remove branch hint
[In reply to]
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On 11/24/2009 12:28 PM, Pekka Enberg wrote:
> On Tue, Nov 24, 2009 at 1:20 PM, Ingo Molnar <mingo [at] elte> wrote:
>> (Pekka Cc:-ed)
>>
>> * Tim Blechmann <tim [at] klingt> 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 [at] klingt>
>>> ---
>>> 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 [at] klingt
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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