From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933829AbeCGPZF (ORCPT ); Wed, 7 Mar 2018 10:25:05 -0500 Received: from usa-sjc-mx-foss1.foss.arm.com ([217.140.101.70]:52780 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933335AbeCGPZD (ORCPT ); Wed, 7 Mar 2018 10:25:03 -0500 Date: Wed, 7 Mar 2018 15:24:58 +0000 From: Patrick Bellasi To: Peter Zijlstra Cc: linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org, Ingo Molnar , "Rafael J . Wysocki" , Viresh Kumar , Vincent Guittot , Paul Turner , Dietmar Eggemann , Morten Rasmussen , Juri Lelli , Todd Kjos , Joel Fernandes , Steve Muckle Subject: Re: [PATCH v5 1/4] sched/fair: add util_est on top of PELT Message-ID: <20180307152458.GE2211@e110439-lin> References: <20180222170153.673-1-patrick.bellasi@arm.com> <20180222170153.673-2-patrick.bellasi@arm.com> <20180306185851.GG25201@hirez.programming.kicks-ass.net> <20180307113149.GA2211@e110439-lin> <20180307122437.GM25181@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180307122437.GM25181@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 07-Mar 13:24, Peter Zijlstra wrote: > On Wed, Mar 07, 2018 at 11:31:49AM +0000, Patrick Bellasi wrote: > > > It appears to me this isn't a stable situation and completely relies on > > > the !nr_running case to recalibrate. If we ensure that doesn't happen > > > for a significant while the sum can run-away, right? > > > > By away you mean go over 1024 or overflow the unsigned int storage? > > the later, I think you can make it arbitrarily large. Have a busy task > on CPU0, this ensure !nr_running never happens. > > Start a busy task on CPU1, wait for it to hit u=1, then migrate it to > CPU0, At this point util_est(CPU0) = 2048, which is: +1024 for the busy running task assuming it has been enqueued with the utilization since the beginning +1024 for the newly migrated task from CPU1 which is enqueued with the value he reached at dequeued time from CPU1 > then wait for it to hit u=.5 then kill it, ... but when we kill it, the task is dequeued, and thus we remove 1024. Maybe that's the tricky bit: we remove the value we enqueued, _not_ the current util_avg. Notice we use _task_util_est(p)... with the leading "_". > this effectively adds > .5 to the enqueued value, repeat indefinitely. Thus this should not happen. Basically, the RQ's util_est is the sum of the RUNNABLE tasks's util_est at their enqueue time... which has been update at their last dequeue time, hence the usage of name "dequeued" for both tasks and rqs. Does it make sense now? -- #include Patrick Bellasi