From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1760072AbaGPBOX (ORCPT ); Tue, 15 Jul 2014 21:14:23 -0400 Received: from cn.fujitsu.com ([59.151.112.132]:65198 "EHLO heian.cn.fujitsu.com" rhost-flags-OK-FAIL-OK-FAIL) by vger.kernel.org with ESMTP id S1757173AbaGPBOU (ORCPT ); Tue, 15 Jul 2014 21:14:20 -0400 X-IronPort-AV: E=Sophos;i="5.00,899,1396972800"; d="scan'208";a="33326708" Message-ID: <53C5D229.7090500@cn.fujitsu.com> Date: Wed, 16 Jul 2014 09:15:21 +0800 From: Lai Jiangshan User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.2.9) Gecko/20100921 Fedora/3.1.4-1.fc14 Thunderbird/3.1.4 MIME-Version: 1.0 To: Tejun Heo CC: Subject: Re: [PATCH RFC] workqueue: don't grab PENDING bit on some conditions References: <1405416610-12394-1-git-send-email-laijs@cn.fujitsu.com> <20140715155805.GD19570@htj.dyndns.org> In-Reply-To: <20140715155805.GD19570@htj.dyndns.org> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit X-Originating-IP: [10.167.226.103] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 07/15/2014 11:58 PM, Tejun Heo wrote: > Hello, Lai. > > On Tue, Jul 15, 2014 at 05:30:10PM +0800, Lai Jiangshan wrote: >> Thread1 expects that, after flush_delayed_work() returns, the known pending >> work is guaranteed finished. But if Thread2 is scheduled a little later than >> Thread1, the known pending work is dequeued and re-queued, it is considered >> as two different works in the workqueue subsystem and the guarantee expected > > They are two separate queueing instances of the same work item. I think the mod_delayed_work() is expected to modify a queueing instances instead of separate from the name. > >> by Thread1 is broken. > > The guarantee expected by thread 1 is that the most recent queueing > instance of the work item is finished either through completing > execution or being cancelled. No guarantee is broken. I don't think the mod_delayed_work() is considered as a cancelling operation to the user. You can add comments to state that it contains a cancelling operation and a requeue operation. > >> The guarantee expected by Thread1/workqueue-user is reasonable for me, >> the workqueue subsystem should provide this guarantee. In another aspect, > > You're adding a new component to the existing set of guarantees. You > can argue for it but it's a new guarantee regardless. So, it is an RFC. > >> the flush_delayed_work() is still working when mod_delayed_work_on() returns, >> it is more acceptable that the flush_delayed_work() beats the >> mod_delayed_work_on(). >> >> It is achieved by introducing a KEEP_FLUSHED flag for try_to_grab_pending(). >> If the work is being flushed and KEEP_FLUSHED flags is set, >> we disallow try_to_grab_pending() to grab the pending of the work. >> >> And there is another condition that the user want to speed up a delayed work. >> >> When the user use "mod_delayed_work_on(..., 0 /* zero delay */);", his >> attention is to accelerate the work and queue the work immediately. >> >> But the work does be slowed down when it is already queued on the worklist >> due to the work is dequeued and re-queued. So we also disallow >> try_to_grab_pending() to grab the pending of the work in this condition >> by introducing KEEP_QUEUED flag. > > Both are extremely marginal. > Do we have any actual cases any of these matters? No such case. I only found the WB subsystem (backing-dev.c, fs-writeback.c) uses both mod_delayed_work() and flush_delayed_work(), but it seems that when flush_delayed_work() is called, mod_delayed_work() will can't be called. > I can't see what we're gaining with the extra complexity. Will you add some comments or let it as before? > >> @@ -1212,6 +1220,13 @@ static int try_to_grab_pending(struct work_struct *work, bool is_dwork, >> */ >> pwq = get_work_pwq(work); >> if (pwq && pwq->pool == pool) { >> + if ((keep_flags | KEEP_QUEUED) || >> + ((keep_flags | KEEP_FLUSHED) && > > This can't be right. > > Thanks. >