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

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Message-Id: <20070409134057.2d249f0c.akpm@linux-foundation.org>
Date:	Mon, 9 Apr 2007 13:40:57 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	"Siddha, Suresh B" <suresh.b.siddha@...el.com>
Cc:	mingo@...e.hu, nickpiggin@...oo.com.au,
	linux-kernel@...r.kernel.org, Andi Kleen <ak@...e.de>,
	Ravikiran G Thirumalai <kiran@...lex86.org>
Subject: Re: [patch] sched: align rq to cacheline boundary

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.

Need more convincing, please.
-
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