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SubjectRe: [PATCH 3/5] slab.c: remove branch hint
Tim Blechmann kirjoitti:
> On 11/24/2009 12:28 PM, Pekka Enberg wrote:
>> On Tue, Nov 24, 2009 at 1:20 PM, Ingo Molnar <mingo@elte.hu> wrote:
>>> (Pekka Cc:-ed)
>>>
>>> * Tim Blechmann <tim@klingt.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@klingt.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 */
>
>
>

OK, so it's the generic alloc_skb() function that keeps hitting
kmem_cache_alloc_node() with "-1". Christoph, are you okay with removing
the unlikely() annotation from __cache_alloc_node()?

Pekka


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