linux-ext4 - Re: [PATCH] percpu_counter: Fix __percpu_counter

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Message-Id: <20081208150724.37761534.akpm@linux-foundation.org>
Date:	Mon, 8 Dec 2008 15:07:24 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Theodore Tso <tytso@....edu>
Cc:	a.p.zijlstra@...llo.nl, dada1@...mosbay.com,
	linux-kernel@...r.kernel.org, davem@...emloft.net, cmm@...ibm.com,
	linux-ext4@...r.kernel.org
Subject: Re: [PATCH] percpu_counter: Fix __percpu_counter_sum()

On Mon, 8 Dec 2008 18:00:47 -0500
Theodore Tso <tytso@....edu> wrote:

> On Mon, Dec 08, 2008 at 11:20:35PM +0100, Peter Zijlstra wrote:
> > 
> > atomic_t is pretty good on all archs, but you get to keep the cacheline
> > ping-pong.
> > 
> 
> Stupid question --- if you're worried about cacheline ping-pongs, why
> aren't each cpu's delta counter cacheline aligned?

They are allocated with alloc_percpu(), so each CPU's counter lives
in a per-cpu area.  If you chase through seventeen layers of Rustyness
you end up at mm/allocpercpu.c:percpu_populate() which is where that
little s32 ends up getting allocated.
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