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

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Message-ID: <84144f020911240328l3d36d347o6c91b2b1a0f50f2a@mail.gmail.com>
Date:	Tue, 24 Nov 2009 13:28:53 +0200
From:	Pekka Enberg <penberg@...helsinki.fi>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Tim Blechmann <tim@...ngt.org>, 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 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.
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