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

[Pekka Enberg]

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@xxxxxxx> wrote:
> > > (Pekka Cc:-ed)
> > >
> > > * Tim Blechmann <tim@xxxxxxxxxx> 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@xxxxxxxxxx>
> > > > ---
> > > >  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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