Messages in this thread | | | Date | Mon, 30 Nov 2009 11:05:06 +0200 | From | Pekka Enberg <> | Subject | Re: [PATCH 3/5] slab.c: remove branch hint |
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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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