From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1763326AbXIXT2X (ORCPT ); Mon, 24 Sep 2007 15:28:23 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1756604AbXIXT2P (ORCPT ); Mon, 24 Sep 2007 15:28:15 -0400 Received: from e4.ny.us.ibm.com ([32.97.182.144]:50691 "EHLO e4.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754901AbXIXT2O (ORCPT ); Mon, 24 Sep 2007 15:28:14 -0400 Subject: Re: [PATCH 24/25] r/o bind mounts: track number of mount writers From: Dave Hansen To: Christoph Hellwig Cc: akpm@osdl.org, linux-kernel@vger.kernel.org In-Reply-To: <20070924175411.GA2314@infradead.org> References: <20070920195249.852667D5@kernel> <20070920195320.38C8E20D@kernel> <20070924175411.GA2314@infradead.org> Content-Type: text/plain Date: Mon, 24 Sep 2007 12:28:11 -0700 Message-Id: <1190662091.26982.246.camel@localhost> Mime-Version: 1.0 X-Mailer: Evolution 2.10.1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Mon, 2007-09-24 at 18:54 +0100, Christoph Hellwig wrote: > As we already say in various messages the percpu counters in here > look rather fishy. I'd recomment to take a look at the per-cpu > superblock counters in XFS as they've been debugged quite well > now and could probably be lifted into a generic library for this > kind of think. The code is mostly in fs/xfs/xfs_mount.c can > can be spotted by beeing under #ifdef HAVE_PERCPU_SB. > > It also handles cases like hotplug cpu nicely that this code > seems to work around by always iterating over all possible cpus > which might not be nice on a dual core laptop with a distro kernel > that also has to support big iron. I'll take a look at xfs to see what I can get out of it. There are basically two times when you have to do this for_each_possible_cpu() stuff: 1. when doing a r/w->r/o transition, which is rare, and certainly not a fast path 2. Where the per-cpu writer count underflows. This requires a _minimum_ of 1<<16 file opens (configurable) each of which is closed on a different cpu than it was opened on. Even if you were trying, I'm not sure you'd notice the overhead. -- Dave