linux-kernel - Re: [patch] sched: align rq to cacheline boundary

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Date:	Mon, 9 Apr 2007 14:53:09 -0700
From:	Ravikiran G Thirumalai <kiran@...lex86.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	"Siddha, Suresh B" <suresh.b.siddha@...el.com>, mingo@...e.hu,
	nickpiggin@...oo.com.au, linux-kernel@...r.kernel.org,
	Andi Kleen <ak@...e.de>
Subject: Re: [patch] sched: align rq to cacheline boundary

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" <suresh.b.siddha@...el.com> 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 <suresh.b.siddha@...el.com>
> > ---
> > 
> > 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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