From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751945AbcFUMsU (ORCPT ); Tue, 21 Jun 2016 08:48:20 -0400 Received: from mga03.intel.com ([134.134.136.65]:24503 "EHLO mga03.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751525AbcFUMsS (ORCPT ); Tue, 21 Jun 2016 08:48:18 -0400 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.26,504,1459839600"; d="scan'208";a="992104891" Date: Tue, 21 Jun 2016 12:51:39 +0800 From: Yuyang Du To: Peter Zijlstra Cc: Dietmar Eggemann , Vincent Guittot , Ingo Molnar , linux-kernel , Mike Galbraith , Benjamin Segall , Paul Turner , Morten Rasmussen , Matt Fleming Subject: Re: [PATCH 4/4] sched,fair: Fix PELT integrity for new tasks Message-ID: <20160621045139.GE19934@intel.com> References: <20160617120136.064100812@infradead.org> <20160617120454.150630859@infradead.org> <20160617142814.GT30154@twins.programming.kicks-ass.net> <20160617160239.GL30927@twins.programming.kicks-ass.net> <20160617161831.GM30927@twins.programming.kicks-ass.net> <5767D51F.3080600@arm.com> <5768027E.1090408@arm.com> <20160621084119.GN30154@twins.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160621084119.GN30154@twins.programming.kicks-ass.net> 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 On Tue, Jun 21, 2016 at 10:41:19AM +0200, Peter Zijlstra wrote: > On Mon, Jun 20, 2016 at 03:49:34PM +0100, Dietmar Eggemann wrote: > > On 20/06/16 13:35, Vincent Guittot wrote: > > > > It will go through wake_up_new_task and post_init_entity_util_avg > > > during its fork which is enough to set last_update_time. Then, it will > > > use the switched_to_fair if the task becomes a fair one > > > > Oh I see. We want to make sure that every task (even when forked as > > !fair) has a last_update_time value != 0, when becoming fair one day. > > Right, see 2 below. I need to write a bunch of comments explaining PELT > proper, as well as document these things. > > The things we ran into with these patches were that: > > 1) You need to update the cfs_rq _before_ any entity attach/detach > (and might need to update_tg_load_avg when update_cfs_rq_load_avg() > returns true). This is intrinsically an additional update, not a fix to anything. I don't think it is a must, but I am fine with it. > 2) (fair) entities are always attached, switched_from/to deal with !fair. Yes, me too. > 3) cpu migration is the only exception and uses the last_update_time=0 > thing -- because refusal to take second rq->lock. Task's last_update_time means this task is detached from fair queue. This (re)definition is by all means much better than migrating. No? > Which is why I dislike Yuyang's patches, they create more exceptions > instead of applying existing rules (albeit undocumented). > > Esp. 1 is important, because while for mathematically consistency you > don't actually need to do this, you only need the entities to be > up-to-date with the cfs rq when you attach/detach, but that forgets the > temporal aspect of _when_ you do this. Yes, temporally at any instant the avgs are outdated. But, I can have it, and what if I have it? I am thinking about document this really well, like "An art of load tracking: accuracy, overhead, and usefulness", seriously.