From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932086Ab0EaIBT (ORCPT ); Mon, 31 May 2010 04:01:19 -0400 Received: from bombadil.infradead.org ([18.85.46.34]:52355 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756421Ab0EaIBE convert rfc822-to-8bit (ORCPT ); Mon, 31 May 2010 04:01:04 -0400 Subject: Re: [PATCH 2/4] sched: implement __set_cpus_allowed() From: Peter Zijlstra To: Tejun Heo Cc: mingo@elte.hu, linux-kernel@vger.kernel.org, Rusty Russell , Mike Galbraith In-Reply-To: <1273747705-7829-3-git-send-email-tj@kernel.org> References: <1273747705-7829-1-git-send-email-tj@kernel.org> <1273747705-7829-3-git-send-email-tj@kernel.org> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8BIT Date: Mon, 31 May 2010 10:01:06 +0200 Message-ID: <1275292866.27810.21441.camel@twins> Mime-Version: 1.0 X-Mailer: Evolution 2.28.3 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 2010-05-13 at 12:48 +0200, Tejun Heo wrote: > Concurrency managed workqueue needs to be able to migrate tasks to a > cpu which is online but !active for the following two purposes. > > p1. To guarantee forward progress during cpu down sequence. Each > workqueue which could be depended upon during memory allocation > has an emergency worker task which is summoned when a pending work > on such workqueue can't be serviced immediately. cpu hotplug > callbacks expect workqueues to work during cpu down sequence > (usually so that they can flush them), so, to guarantee forward > progress, it should be possible to summon emergency workers to > !active but online cpus. If we do the thing suggested in the previous patch, that is move clearing active and rebuilding the sched domains until right after DOWN_PREPARE, this goes away, right? > p2. To migrate back unbound workers when a cpu comes back online. > When a cpu goes down, existing workers are unbound from the cpu > and allowed to run on other cpus if there still are pending or > running works. If the cpu comes back online while those workers > are still around, those workers are migrated back and re-bound to > the cpu. This isn't strictly required for correctness as long as > those unbound workers don't execute works which are newly > scheduled after the cpu comes back online; however, migrating back > the workers has the advantage of making the behavior more > consistent thus avoiding surprises which are difficult to expect > and reproduce, and being actually cleaner and easier to implement. I still don't like this much, if you mark these tasks to simply die when the queue is exhausted, and flush the queue explicitly on CPU_UP_PREPARE, you should never need to do this. After which I think you don't need this patch anymore..