On Mon, Apr 09, 2007 at 01:40:57PM -0700, Andrew Morton wrote:
> On Mon, 9 Apr 2007 11:08:53 -0700
> "Siddha, Suresh B" <[email protected]> wrote:
>
> > Align the per cpu runqueue to the cacheline boundary. This will minimize the
> > number of cachelines touched during remote wakeup.
> >
> > Signed-off-by: Suresh Siddha <[email protected]>
> > ---
> >
> > diff --git a/kernel/sched.c b/kernel/sched.c
> > index b9a6837..eca33c5 100644
> > --- a/kernel/sched.c
> > +++ b/kernel/sched.c
> > @@ -278,7 +278,7 @@ struct rq {
> > struct lock_class_key rq_lock_key;
> > };
> >
> > -static DEFINE_PER_CPU(struct rq, runqueues);
> > +static DEFINE_PER_CPU(struct rq, runqueues) ____cacheline_aligned_in_smp;
>
> Remember that this can consume up to (linesize-4 * NR_CPUS) bytes, which is
> rather a lot.
>
> Remember also that the linesize on VSMP is 4k.
>
> And that putting a gap in the per-cpu memory like this will reduce its
> overall cache-friendliness.
>
The internode line size yes. But Suresh is using ____cacheline_aligned_in_smp,
which uses SMP_CACHE_BYTES (L1_CACHE_BYTES). So this does not align the
per-cpu variable to 4k. However, if the motivation for this patch was
significant performance difference, then, the above padding needs to be on
the internode cacheline size using ____cacheline_internodealigned_in_smp.
____cacheline_internodealigned_in_smp aligns a data structure to the
internode line size, which is 4k for vSMPowered systems and L1 line size
for all other architectures.
As for the (linesize-4 * NR_CPUS) wastage, maybe we can place the cacheline
aligned per-cpu data in another section, just like we do with
.data.cacheline_aligned section, but keep this new section between
__percpu_start and __percpu_end?
-
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