From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753679AbaETOfj (ORCPT ); Tue, 20 May 2014 10:35:39 -0400 Received: from mail-qg0-f45.google.com ([209.85.192.45]:44253 "EHLO mail-qg0-f45.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750808AbaETOfi (ORCPT ); Tue, 20 May 2014 10:35:38 -0400 Date: Tue, 20 May 2014 10:35:34 -0400 From: Tejun Heo To: Frederic Weisbecker Cc: LKML , Lai Jiangshan , Christoph Lameter , Kevin Hilman , Mike Galbraith , "Paul E. McKenney" , Viresh Kumar Subject: Re: [PATCH 1/5] workqueue: Allow changing attributions of ordered workqueues Message-ID: <20140520143534.GD2804@htj.dyndns.org> References: <1400257015-28222-1-git-send-email-fweisbec@gmail.com> <1400257015-28222-2-git-send-email-fweisbec@gmail.com> <20140516201225.GL5379@htj.dyndns.org> <20140517134151.GA32371@localhost.localdomain> <20140519201531.GC27506@mtj.dyndns.org> <20140520143229.GB17741@localhost.localdomain> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140520143229.GB17741@localhost.localdomain> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hello, On Tue, May 20, 2014 at 04:32:31PM +0200, Frederic Weisbecker wrote: > > But that's the same for other pwqs too. Back-to-back requeueing will > > hold back pwq switching on any workqueue. > > I don't think so, because non ordered pwqs aren't created with 0 max_active, > so they can run before the old pwq is released. It's not holding back the new > one and creating a starvation there. > > But maybe I forget other details. Ah, I was thinking about old pwq not being allowed to be released while one or more work items are requeueing themselves back-to-back. Yeap, the new ones can still be used for other work items. > > Maybe I'm confused but I don't think it'd be. Let's say there was an > > attribute change with one work item, A, which is performing > > back-to-back requeueing and another one, B, which queues itself > > intermittently. If B is queued while A is executing, followed by A > > requeueing itself, the expected execution order is A - B - A; however, > > without the above exception for ordered workqueues, it'd end up A - A > > - B because B will end up on the new pwq while A on the older one and > > max_active won't be transferred to the new pwq before it becomes > > empty. > > Ah right AAB instead of ABA is possible indeed. I don't know if some workqueue > rely on such guarantee but it's possible. That's part of the ordering guarantee of ordered workqueues so we better not break it. > In which case we have one more reason to make an exception on ordered workqueues > previous pwq reuse. Yeah, I agree the special treatment is necessary but the current comment is misleading. Thanks. -- tejun