linux-kernel - Re: [PATCH] Move mostly read variables to __read

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Date:	Sun, 23 Oct 2011 21:47:45 -0700
From:	Andi Kleen <andi@...stfloor.org>
To:	Eial Czerwacki <eial@...lemp.com>
Cc:	linux-kernel@...r.kernel.org,
	"Shai Fultheim \(Shai\@ScaleMP.com\)" <Shai@...lemp.com>
Subject: Re: [PATCH] Move mostly read variables to __read_mostly section.

Eial Czerwacki <eial@...lemp.com> writes:
>   * Once the number of bh's in the machine exceeds this level, we start
> --- a/kernel/time/timekeeping.c	2010-05-12 14:59:32.000000000 -0700
> +++ b/kernel/time/timekeeping.c	2010-05-24 11:56:32.000000000 -0700
> @@ -154,7 +154,7 @@ __cacheline_aligned_in_smp DEFINE_SEQLOC
>   * used instead.
>   */
>  static struct timespec xtime __attribute__ ((aligned (16)));
> -static struct timespec wall_to_monotonic __attribute__ ((aligned (16)));
> +static struct timespec wall_to_monotonic __attribute__ ((aligned
>   (16))) __read_mostly;

That means that a common time fetch/update call will usually have to fetch two
cachelines now instead of one, because it needs xtime and
wall_to_monotonic. Usually those end up nearby.

On the other hand it's rare to fetch wall_to_monotonic without xtime.

So if anything all those variables should be grouped together in a
single cacheline, but definitely not split like you do.

The other changes in your patch are fine for me.

-Andi
-- 
ak@...ux.intel.com -- Speaking for myself only
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