Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754852AbYKJOcv (ORCPT ); Mon, 10 Nov 2008 09:32:51 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752778AbYKJOco (ORCPT ); Mon, 10 Nov 2008 09:32:44 -0500 Received: from mx1.redhat.com ([66.187.233.31]:46223 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752724AbYKJOcn (ORCPT ); Mon, 10 Nov 2008 09:32:43 -0500 Date: Mon, 10 Nov 2008 09:32:27 -0500 (EST) From: Mikulas Patocka X-X-Sender: mpatocka@hs20-bc2-1.build.redhat.com To: Peter Zijlstra cc: Christoph Hellwig , Chandra Seetharaman , Alasdair G Kergon , dm-devel , linux-kernel@vger.kernel.org, axboe@kernel.dk Subject: Re: Queue upcall locking (was: [dm-devel] [RFC][PATCH] fix dm_any_congested() to properly sync up with suspend code path) In-Reply-To: <1226327210.7685.173.camel@twins> Message-ID: References: <1225944008.14830.1101.camel@chandra-ubuntu> <20081110135401.GA11606@infradead.org> <1226327210.7685.173.camel@twins> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2131 Lines: 46 On Mon, 10 Nov 2008, Peter Zijlstra wrote: > On Mon, 2008-11-10 at 09:19 -0500, Mikulas Patocka wrote: > > On Mon, 10 Nov 2008, Christoph Hellwig wrote: > > > > > On Mon, Nov 10, 2008 at 08:11:51AM -0500, Mikulas Patocka wrote: > > > > For upstream Linux developers: you are holding a spinlock and calling > > > > bdi*_congested functions that can take indefinite amount of time (there > > > > are even users reporting having 50 disks in one logical volume or so). I > > > > think it would be good to move these calls out of spinlocks. > > > > > > Umm, they shouldn't block that long, as that completely defeats their > > > purpose. These functions are mostly used to avoid throwing more I/O at > > > a congested device if pdflush could do more useful things instead. But > > > if it blocks in those functions anyway we wouldn't have to bother using > > > them. Do you have more details about the uses cases when this happens > > > and where the routines spend so much time? > > > > For device mapper, congested_fn asks every device in the tree and make OR > > of their bits --- so if the user has 50 devices, it asks them all. > > > > For md-linear, md-raid0, md-raid1, md-raid10 and md-multipath it does the > > same --- asking every device. > > > > If you have a better idea how to implement congested_fn, say it. > > Fix the infrastructure by adding a function call so that you can have > the individual devices report their congestion state to the aggregate. > > Then congestion_fn can return a valid state in O(1) because the state is > keps up-to-date by the individual state changes. > > IOW, add a set_congested_fn() and clear_congested_fn(). If you have a physical disk that has many LVM volumes on it, you end up in a situation when disk congestion state change is reported to all the volumes. So it will create O(n) problem at the other side. Mikulas -- 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/