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

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Message-Id: <1228777687.12729.6.camel@twins>
Date:	Tue, 09 Dec 2008 00:08:07 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Theodore Tso <tytso@....edu>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Eric Dumazet <dada1@...mosbay.com>,
	linux kernel <linux-kernel@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Mingming Cao <cmm@...ibm.com>, linux-ext4@...r.kernel.org
Subject: Re: [PATCH] percpu_counter: Fix __percpu_counter_sum()

On Tue, 2008-12-09 at 00:05 +0100, Peter Zijlstra wrote:
> On Mon, 2008-12-08 at 18:00 -0500, Theodore Tso 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?  With a 64-byte
> > cache-line, and a 32-bit counters entry, with less than 16 CPU's we're
> > going to be getting cache ping-pong effects with percpu_counter's,
> > right?  Or am I missing something?
> 
> sorta - a new per-cpu allocator is in the works, but we do cacheline
> align the per-cpu allocations (or used to), also, the allocations are
> node affine.

Indeed we still (or again) do, see mm/allocpercpu.c:percpu_populate().

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