Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756746Ab0HCOM2 (ORCPT ); Tue, 3 Aug 2010 10:12:28 -0400 Received: from smtp.nokia.com ([192.100.105.134]:24382 "EHLO mgw-mx09.nokia.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756585Ab0HCOM0 (ORCPT ); Tue, 3 Aug 2010 10:12:26 -0400 Subject: Re: [PATCHv6 00/15] kill unnecessary bdi wakeups + cleanups From: Artem Bityutskiy Reply-To: dedekind1@gmail.com To: Jens Axboe Cc: linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org In-Reply-To: <4C580B1C.3020409@kernel.dk> References: <1280057365-10297-1-git-send-email-dedekind1@gmail.com> <4C580B1C.3020409@kernel.dk> Content-Type: text/plain; charset="UTF-8" Date: Tue, 03 Aug 2010 17:11:21 +0300 Message-ID: <1280844681.15689.59.camel@localhost> Mime-Version: 1.0 X-Mailer: Evolution 2.30.2 (2.30.2-4.fc13) Content-Transfer-Encoding: 8bit X-OriginalArrivalTime: 03 Aug 2010 14:12:19.0266 (UTC) FILETIME=[E2353A20:01CB3315] X-Nokia-AV: Clean Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2426 Lines: 58 On Tue, 2010-08-03 at 14:27 +0200, Jens Axboe wrote: > On 2010-07-25 13:29, Artem Bityutskiy wrote: > > Hi, > > > > here is v6 of the patch series which clean-ups bdi threads and substantially > > lessens amount of unnecessary kernel wake-ups, which is very important on > > battery-powered devices. > > > > This patch-set is also available at: > > git://git.infradead.org/users/dedekind/misc-2.6.git flushers_v6 > > Thanks Artem, for sticking around long enough to get this into > shape. I have finally merged it. > > > 1. Use 'spin_lock_bh' for the 'bdi->wb_lock' (changed patch N12) > > I'd rather not, question is how to avoid it. Either just wakeup the > default thread, or punt the lock-and-check bdi->wb.task to a thread. Jens, here are my quick thoughts, will come back to this tomorrow. The spin_lock_bh(&bdi->wb_lock) in 'wakeup_timer_fn()' is needed: a) to make sure the forker thread does not decide to kill the bdi thread at the same time, which could cause an oops on 'wake_up_process(bdi->wb.task)'. b) to make sure the forker thread does not decide to spawn a bdi thread at the same time, in which case we could lose a wake-up. I without the "_bh" suffix lockdep complains with a warning. Cannot cite the complained, but it is a fair warning about a possible deadlock if the timer function interrupts the CPU while it is already holding the spinlock, or something like that. The easiest way to address it was to use "_bh". The only way to avoid "_bh" I see right now is to not 'bdi->wb_lock' at all in 'wakeup_timer_fn()'. In this case we cannot touch 'bdi->wb.task' because it can become NULL at any point of time. Your first suggestion ("just wakeup the default thread") will work only if we add a new BDI_wakeup_thread or something like that. Not sure it is worth it. The second suggestion ("punt the lock-and-check bdi->wb.task to a thread") is vague. "A thread" - this must be the forker thread, what else could that be? So basically this is the same as the first suggestion - we set a flag in 'bdi->wb.state' and wake up the forker, which should wake up the bdi thread? -- Best Regards, Artem Bityutskiy (Артём Битюцкий) -- 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/