From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933939AbeCGPhi (ORCPT ); Wed, 7 Mar 2018 10:37:38 -0500 Received: from foss.arm.com ([217.140.101.70]:53028 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933487AbeCGPhh (ORCPT ); Wed, 7 Mar 2018 10:37:37 -0500 Date: Wed, 7 Mar 2018 15:37:32 +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: <20180307153732.GF2211@e110439-lin> References: <20180222170153.673-1-patrick.bellasi@arm.com> <20180222170153.673-2-patrick.bellasi@arm.com> <20180306185851.GG25201@hirez.programming.kicks-ass.net> <20180307093937.GZ25235@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180307093937.GZ25235@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 10:39, Peter Zijlstra wrote: > On Tue, Mar 06, 2018 at 07:58:51PM +0100, Peter Zijlstra wrote: > > On Thu, Feb 22, 2018 at 05:01:50PM +0000, Patrick Bellasi wrote: > > > +static inline void util_est_enqueue(struct cfs_rq *cfs_rq, > > > + struct task_struct *p) > > > +{ > > > + unsigned int enqueued; > > > + > > > + if (!sched_feat(UTIL_EST)) > > > + return; > > > + > > > + /* Update root cfs_rq's estimated utilization */ > > > + enqueued = READ_ONCE(cfs_rq->avg.util_est.enqueued); > > > + enqueued += _task_util_est(p); > > > + WRITE_ONCE(cfs_rq->avg.util_est.enqueued, enqueued); > > > +} > > > 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? > > > > Should we put a max in enqueue to avoid this? > > Thinking about this a bit more; would it make sense to adjust the > running sum/avg on migration? Something along the lines of: > > util_avg = se->load_avg / (cfs_rq->load_avg + se->load_avg); > > (which disregards cgroups), because that should more or less be the time > it ends up running, given the WFQ rule. I would say it makes sense from a purely mechanism stanpoing, but I'm not entirely convinced it can be useful from a practical stanpoint. First of all, that should be applied only when we migrate to a more saturated CPU. Otherwise, when migrating on an empty CPU we would set util_avg = 100% Secondly, when we migrate to a saturated CPU, this adjustment will contribute to under-estimate the task utilization. Let say the task was running on a completely empty CPU, and thus we was able to ramp up without being preempted. This value represents a good estimation of the (most recent) task CPU demands. Now, if on a following activation, we wakeup the task on an IDLE CPU with a lot of blocked load, then we will scale down its util_avg and assume the task will be smaller. But: a) if the blocked load does not turns into some task waking up again, underestimated the task introduces only further ramp-up latencies b) if the load it due to really active tasks, the task will be preempted and it's utilization smaller... but we are already in a domain where utilization does not tell us anything useful for a task... and thus, why bothering to make it converging sooner? > That way the disparity between tasks migrating into the CPU at u=1 and > them going to sleep at u<1 is much smaller and the above sum doesn't run > away nearly as wild (it still needs some upper bound though). -- #include Patrick Bellasi