Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755571AbYH1EKs (ORCPT ); Thu, 28 Aug 2008 00:10:48 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754914AbYH1EKe (ORCPT ); Thu, 28 Aug 2008 00:10:34 -0400 Received: from smtp1.linux-foundation.org ([140.211.169.13]:39739 "EHLO smtp1.linux-foundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755414AbYH1EKb (ORCPT ); Thu, 28 Aug 2008 00:10:31 -0400 Date: Wed, 27 Aug 2008 21:09:25 -0700 From: Andrew Morton To: "Aneesh Kumar K.V" Cc: Peter Zijlstra , cmm@us.ibm.com, tytso@mit.edu, sandeen@redhat.com, linux-ext4@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH -V3 01/11] percpu_counters: make fbc->count read atomic on 32 bit architecture Message-Id: <20080827210925.b4846037.akpm@linux-foundation.org> In-Reply-To: <20080828035200.GB6440@skywalker> References: <1219850916-8986-1-git-send-email-aneesh.kumar@linux.vnet.ibm.com> <20080827120553.9c9d6690.akpm@linux-foundation.org> <1219870912.6395.45.camel@twins> <20080827142250.7397a1a7.akpm@linux-foundation.org> <20080828035200.GB6440@skywalker> X-Mailer: Sylpheed 2.4.8 (GTK+ 2.12.5; x86_64-redhat-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2496 Lines: 57 On Thu, 28 Aug 2008 09:22:00 +0530 "Aneesh Kumar K.V" wrote: > On Wed, Aug 27, 2008 at 02:22:50PM -0700, Andrew Morton wrote: > > On Wed, 27 Aug 2008 23:01:52 +0200 > > Peter Zijlstra wrote: > > > > > > > > > > > +static inline s64 percpu_counter_read(struct percpu_counter *fbc) > > > > > +{ > > > > > + return fbc_count(fbc); > > > > > +} > > > > > > > > This change means that a percpu_counter_read() from interrupt context > > > > on a 32-bit machine is now deadlockable, whereas it previously was not > > > > deadlockable on either 32-bit or 64-bit. > > > > > > > > This flows on to the lib/proportions.c, which uses > > > > percpu_counter_read() and also does spin_lock_irqsave() internally, > > > > indicating that it is (or was) designed to be used in IRQ contexts. > > > > > > percpu_counter() never was irq safe, which is why the proportion stuff > > > does all the irq disabling bits by hand. > > > > percpu_counter_read() was irq-safe. That changes here. Needs careful > > review, changelogging and, preferably, runtime checks. But perhaps > > they should be inside some CONFIG_thing which won't normally be done in > > production. > > > > otoh, percpu_counter_read() is in fact a rare operation, so a bit of > > overhead probably won't matter. > > > > (write-often, read-rarely is the whole point. This patch's changelog's > > assertion that "Since fbc->count is read more frequently and updated > > rarely" is probably wrong. Most percpu_counters will have their > > fbc->count modified far more frequently than having it read from). > > we may actually be doing percpu_counter_add. But that doesn't update > fbc->count. Only if the local percpu values cross FBC_BATCH we update > fbc->count. If we are modifying fbc->count more frequently than > reading fbc->count then i guess we would be contenting of fbc->lock more. > > Yep. The frequency of modification of fbc->count is of the order of a tenth or a hundredth of the frequency of precpu_counter_() calls. But in many cases the frequency of percpu_counter_read() calls is far far less than this. For example, the percpu_counter_read() may only happen when userspace polls a /proc file. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/