From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-3.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,URIBL_BLOCKED,USER_AGENT_NEOMUTT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 3FF63C43142 for ; Thu, 2 Aug 2018 14:14:36 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id E67CD2150E for ; Thu, 2 Aug 2018 14:14:35 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org E67CD2150E Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732517AbeHBQF5 (ORCPT ); Thu, 2 Aug 2018 12:05:57 -0400 Received: from foss.arm.com ([217.140.101.70]:58748 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1732255AbeHBQF5 (ORCPT ); Thu, 2 Aug 2018 12:05:57 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.72.51.249]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 2F2327A9; Thu, 2 Aug 2018 07:14:33 -0700 (PDT) Received: from queper01-lin (queper01-lin.emea.arm.com [10.4.13.27]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 37B3B3F5B3; Thu, 2 Aug 2018 07:14:29 -0700 (PDT) Date: Thu, 2 Aug 2018 15:14:24 +0100 From: Quentin Perret To: Vincent Guittot Cc: Peter Zijlstra , "Rafael J. Wysocki" , linux-kernel , "open list:THERMAL" , "gregkh@linuxfoundation.org" , Ingo Molnar , Dietmar Eggemann , Morten Rasmussen , Chris Redpath , Patrick Bellasi , Valentin Schneider , Thara Gopinath , viresh kumar , Todd Kjos , Joel Fernandes , "Cc: Steve Muckle" , adharmap@quicinc.com, "Kannan, Saravana" , pkondeti@codeaurora.org, Juri Lelli , Eduardo Valentin , Srinivas Pandruvada , currojerez@riseup.net, Javi Merino Subject: Re: [PATCH v5 09/14] sched: Add over-utilization/tipping point indicator Message-ID: <20180802141424.ju4jxxbk6pxw3kyq@queper01-lin> References: <20180724122521.22109-1-quentin.perret@arm.com> <20180724122521.22109-10-quentin.perret@arm.com> <20180802122629.GU2476@hirez.programming.kicks-ass.net> <20180802130337.uf6tlac2hg4nkbwr@queper01-lin> <20180802130801.GL2494@hirez.programming.kicks-ass.net> <20180802131849.mqpt5lbtcqrxbwig@queper01-lin> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: NeoMutt/20171215 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thursday 02 Aug 2018 at 15:48:01 (+0200), Vincent Guittot wrote: > On Thu, 2 Aug 2018 at 15:19, Quentin Perret wrote: > > > > On Thursday 02 Aug 2018 at 15:08:01 (+0200), Peter Zijlstra wrote: > > > On Thu, Aug 02, 2018 at 02:03:38PM +0100, Quentin Perret wrote: > > > > On Thursday 02 Aug 2018 at 14:26:29 (+0200), Peter Zijlstra wrote: > > > > > On Tue, Jul 24, 2018 at 01:25:16PM +0100, Quentin Perret wrote: > > > > > > @@ -5100,8 +5118,17 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags) > > > > > > update_cfs_group(se); > > > > > > } > > > > > > > > > > > > - if (!se) > > > > > > + if (!se) { > > > > > > add_nr_running(rq, 1); > > > > > > + /* > > > > > > + * The utilization of a new task is 'wrong' so wait for it > > > > > > + * to build some utilization history before trying to detect > > > > > > + * the overutilized flag. > > > > > > + */ > > > > > > + if (flags & ENQUEUE_WAKEUP) > > > > > > + update_overutilized_status(rq); > > > > > > + > > > > > > + } > > > > > > > > > > > > hrtick_update(rq); > > > > > > } > > > > > > > > > > That is a somewhat dodgy hack. There is no guarantee what so ever that > > > > > when the task wakes next its history is any better. The comment doesn't > > > > > reflect this I feel. > > > > > > > > AFAICT the main use-case here is to avoid re-enabling the load balance > > > > and ruining all the task placement because of a tiny task. I don't > > > > really see how we can do that differently ... > > > > > > Sure I realize that.. but it doesn't completely avoid it. Suppose this > > > new task instantly blocks and wakes up again. Then its util signal will > > > be exactly what you didn't want but we'll account it and cause the above > > > scenario you wanted to avoid. > > > > That is true. ... I also realize now that this patch was written long > > before util_est, and that also has an impact here, especially in the > > scenario you described where the task blocks. So any wake-up after the > > first enqueue will risk to overutilize the system, even if the task > > blocked for ages. > > > > Hmm ... > > Does a init value set to 0 for util_avg for newly created task can > help in EAS in this case ? > Current initial value is computed to prevent packing newly created > tasks on same CPUs because it hurts performance of some benches. In > fact it somehow assumes that newly created task will use significant > part of the remaining capacity of a CPU and want to spread tasks. In > EAS case, it seems that it prefer to assume that newly created task > are small and we can pack them and wait a bit to make sure the new > task will be a big task and will overload the CPU Good point, setting the util_avg to 0 for new tasks should help filtering out those tiny tasks too. And that would match with the idea of letting tasks build their history before looking at their util_avg ... But there is one difference w.r.t frequency selection. The current code won't mark the system overutilized, but will let sugov raise the frequency when a new task is enqueued. So in case of a fork bomb, we sort of fallback on the existing mainline strategy for both task placement (because forkees don't go in find_energy_efficient_cpu) and frequency selection. And I would argue this is the right thing to do since EAS can't really help in this case. Thoughts ? Thanks, Quentin