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

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Message-ID: <20091124114218.GA24396@elte.hu>
Date:	Tue, 24 Nov 2009 12:42:18 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Pekka Enberg <penberg@...helsinki.fi>
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


* Pekka Enberg <penberg@...helsinki.fi> 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.

I think it could occur in too limited tests - the branch prediction 
looks 'wrong' in certain tests - while it's OK in general.

Is there some easy to run workload you consider more or less 
representative of typical SLAB patterns?

<plug> You might want to pull even with the scheduler subsystem and in 
       addition to 'perf bench sched', add a 'perf bench slab' set of 
       interesting testcases for SLAB performance testing. :-) 
</plug>

	Ingo
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