From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751979AbcFGDCb (ORCPT ); Mon, 6 Jun 2016 23:02:31 -0400 Received: from mga14.intel.com ([192.55.52.115]:33646 "EHLO mga14.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751091AbcFGDCa (ORCPT ); Mon, 6 Jun 2016 23:02:30 -0400 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.26,431,1459839600"; d="scan'208";a="996619221" Date: Tue, 7 Jun 2016 03:05:24 +0800 From: Yuyang Du To: Vincent Guittot Cc: Peter Zijlstra , Ingo Molnar , linux-kernel , Benjamin Segall , Paul Turner , Morten Rasmussen , Dietmar Eggemann Subject: Re: [PATCH v4 2/5] sched/fair: Fix attaching task sched avgs twice when switching to fair or changing task group Message-ID: <20160606190524.GD8105@intel.com> References: <1465172441-27727-1-git-send-email-yuyang.du@intel.com> <1465172441-27727-3-git-send-email-yuyang.du@intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Vincent, On Mon, Jun 06, 2016 at 02:32:39PM +0200, Vincent Guittot wrote: > > Unlike the switch to fair class case, if the task is on_rq, it will be > > enqueued after we move task groups, so the simplest solution is to reset > > the task's last_update_time when we do task_move_group(), but not to > > attach sched avgs in task_move_group(), and then let enqueue_task() do > > the sched avgs attachment. > > According to the review of the previous version > http://www.gossamer-threads.com/lists/linux/kernel/2450678#2450678, > only the use case switch to fair with a task that has never been > queued as a CFS task before, can have the issue and this will not > happen for other use cases described above. > So can you limit the description in the commit message to just this > use case unless you have discovered new use cases and in this case > please add the description. I assure you that I will, :) > Then, the problem with this use case, comes that last_update_time == 0 > has a special meaning ( task has migrated ) and we initialize > last_update_time with this value. In general, the meaning of last_update_time == 0 is: not yet attached to any cfs queue. > A much more simple solution would be > to prevent last_update_time to be initialized with this special value. > We can initialize the last_update_time of a sched_entity to 1 as an > example which is easier than these changes. Then at least, we must differentiate CONFIG_FAIR_GROUP_SCHED and !CONFIG_FAIR_GROUP_SCHED. And I am not sure whether this is a simpler solution, as I haven't sorted out the whole thing. IMO, this solution seems a little too hacky and I'd prefer not if we have alternative.