Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755529AbYH1Dwf (ORCPT ); Wed, 27 Aug 2008 23:52:35 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754937AbYH1DwM (ORCPT ); Wed, 27 Aug 2008 23:52:12 -0400 Received: from E23SMTP06.au.ibm.com ([202.81.18.175]:41586 "EHLO e23smtp06.au.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754082AbYH1DwL (ORCPT ); Wed, 27 Aug 2008 23:52:11 -0400 Date: Thu, 28 Aug 2008 09:22:00 +0530 From: "Aneesh Kumar K.V" To: Andrew Morton 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: <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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20080827142250.7397a1a7.akpm@linux-foundation.org> User-Agent: Mutt/1.5.17+20080114 (2008-01-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1992 Lines: 46 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. -aneesh -- 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/