From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756244Ab0EaIAz (ORCPT ); Mon, 31 May 2010 04:00:55 -0400 Received: from casper.infradead.org ([85.118.1.10]:48936 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751132Ab0EaIAy convert rfc822-to-8bit (ORCPT ); Mon, 31 May 2010 04:00:54 -0400 Subject: Re: [PATCH 1/4] sched: consult online mask instead of active in select_fallback_rq() From: Peter Zijlstra To: Tejun Heo Cc: mingo@elte.hu, linux-kernel@vger.kernel.org In-Reply-To: <1273747705-7829-2-git-send-email-tj@kernel.org> References: <1273747705-7829-1-git-send-email-tj@kernel.org> <1273747705-7829-2-git-send-email-tj@kernel.org> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8BIT Date: Mon, 31 May 2010 10:01:04 +0200 Message-ID: <1275292864.27810.21440.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: > If called after sched_class chooses a CPU which isn't in a task's > cpus_allowed mask, select_fallback_rq() can end up migrating a task > which is bound to an !active but online cpu to an active cpu. This is > dangerous because active is cleared before CPU_DOWN_PREPARE is called > and subsystems expect affinities of kthreads and other tasks to be > maintained till their CPU_DOWN_PREPARE callbacks are complete. So 6ad4c188 (sched: Fix balance vs hotplug race) moved it that early because it was done too late. Could we not instead do it explicitly after CPU_DOWN_PREPARE? It would of course mean removing the partition_sched_domain() and generate_sched_domains() calls from these callbacks and doing it explicitly. So we need to do it before we take the CPU down, but I think we can do it after DOWN_PREPARE. > Consult cpu_online_mask instead. > > This problem is triggered by cmwq. During CPU_DOWN_PREPARE, hotplug > callback creates the trustee kthread and kthread_bind()s it to the > target cpu, and the trustee is expected to run on that cpu. It doesn't explain wth a trustee kthread is, which pretty much renders the whole paragraph useless.