Date: Mon, 8 Dec 2008 15:07:24 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Theodore Tso <tytso@mit.edu>
Cc: a.p.zijlstra@chello.nl, dada1@cosmosbay.com, linux-kernel@vger.kernel.org,
       davem@davemloft.net, cmm@us.ibm.com, linux-ext4@vger.kernel.org
Subject: Re: [PATCH] percpu_counter: Fix __percpu_counter_sum()
Message-Id: <20081208150724.37761534.akpm@linux-foundation.org>
In-Reply-To: <20081208230047.GC2501@mit.edu>
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On Mon, 8 Dec 2008 18:00:47 -0500
Theodore Tso <tytso@mit.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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