From: Theodore Tso Subject: Re: [PATCH] percpu_counter: Fix __percpu_counter_sum() Date: Mon, 8 Dec 2008 18:49:27 -0500 Message-ID: <20081208234927.GF2501@mit.edu> References: <4936EB04.8000609@cosmosbay.com> <20081206202233.3b74febc.akpm@linux-foundation.org> <493BCF60.1080409@cosmosbay.com> <20081207092854.f6bcbfae.akpm@linux-foundation.org> <493C0F40.7040304@cosmosbay.com> <20081207205250.dbb7fe4b.akpm@linux-foundation.org> <20081208221241.GA2501@mit.edu> <1228774836.16244.22.camel@lappy.programming.kicks-ass.net> <20081208230047.GC2501@mit.edu> <20081208150724.37761534.akpm@linux-foundation.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii 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 To: Andrew Morton Return-path: Received: from www.church-of-our-saviour.org ([69.25.196.31]:37386 "EHLO thunker.thunk.org" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1752134AbYLHXtl (ORCPT ); Mon, 8 Dec 2008 18:49:41 -0500 Content-Disposition: inline In-Reply-To: <20081208150724.37761534.akpm@linux-foundation.org> Sender: linux-ext4-owner@vger.kernel.org List-ID: On Mon, Dec 08, 2008 at 03:07:24PM -0800, Andrew Morton wrote: > > 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. Ah, OK. So the answer is "Ted was stupid and didn't understand n layers of percpu abstractions".... whoever wrote that code seems to have been a great believer of the saying, "if the code was hard to *write*, it should be hard to *understand*". :-) - Ted