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=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no 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 59E7AC3A5A5 for ; Thu, 5 Sep 2019 11:46:45 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 32A2220870 for ; Thu, 5 Sep 2019 11:46:45 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732477AbfIELqn (ORCPT ); Thu, 5 Sep 2019 07:46:43 -0400 Received: from foss.arm.com ([217.140.110.172]:43226 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730780AbfIELqn (ORCPT ); Thu, 5 Sep 2019 07:46:43 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 7FB3B28; Thu, 5 Sep 2019 04:46:40 -0700 (PDT) Received: from [10.1.194.37] (e113632-lin.cambridge.arm.com [10.1.194.37]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id C3B523F718; Thu, 5 Sep 2019 04:46:38 -0700 (PDT) Subject: Re: [RFC PATCH 1/9] sched,cgroup: Add interface for latency-nice To: Patrick Bellasi , Peter Zijlstra Cc: Subhra Mazumdar , linux-kernel@vger.kernel.org, mingo@redhat.com, tglx@linutronix.de, steven.sistare@oracle.com, dhaval.giani@oracle.com, daniel.lezcano@linaro.org, vincent.guittot@linaro.org, viresh.kumar@linaro.org, tim.c.chen@linux.intel.com, mgorman@techsingularity.net, parth@linux.ibm.com References: <20190830174944.21741-1-subhra.mazumdar@oracle.com> <20190830174944.21741-2-subhra.mazumdar@oracle.com> <20190905083127.GA2332@hirez.programming.kicks-ass.net> <87r24v2i14.fsf@arm.com> <20190905104616.GD2332@hirez.programming.kicks-ass.net> <87imq72dpc.fsf@arm.com> From: Valentin Schneider Message-ID: Date: Thu, 5 Sep 2019 12:46:37 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0 MIME-Version: 1.0 In-Reply-To: <87imq72dpc.fsf@arm.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 05/09/2019 12:18, Patrick Bellasi wrote: >> There's a few things wrong there; I really feel that if we call it nice, >> it should be like nice. Otherwise we should call it latency-bias and not >> have the association with nice to confuse people. >> >> Secondly; the default should be in the middle of the range. Naturally >> this would be a signed range like nice [-(x+1),x] for some x. but if you >> want [0,1024], then the default really should be 512, but personally I >> like 0 better as a default, in which case we need negative numbers. >> >> This is important because we want to be able to bias towards less >> importance to (tail) latency as well as more importantance to (tail) >> latency. >> >> Specifically, Oracle wants to sacrifice (some) latency for throughput. >> Facebook OTOH seems to want to sacrifice (some) throughput for latency. > > Right, we have this dualism to deal with and current mainline behaviour > is somehow in the middle. > > BTW, the FB requirement is the same we have in Android. > We want some CFS tasks to have very small latency and a low chance > to be preempted by the wake-up of less-important "background" tasks. > > I'm not totally against the usage of a signed range, but I'm thinking > that since we are introducing a new (non POSIX) concept we can get the > chance to make it more human friendly. > > Give the two extremes above, would not be much simpler and intuitive to > have 0 implementing the FB/Android (no latency) case and 1024 the > (max latency) Oracle case? > For something like latency-, I don't see the point of having such a wide range. The nice range is probably more than enough - and before even bothering about the range, we should probably agree on what the range should represent. If it's niceness, I read it as: positive latency-nice value means we're nice to latency, means we reduce it. So the further up you go, the more you restrict your wakeup scan. I think it's quite easy to map that into the code: current behaviour at 0, with a decreasing scan mask size as we go towards +19. I don't think anyone needs 512 steps to tune this. I don't know what logic we'd follow for negative values though. Maybe latency-nice -20 means always going through the slowpath, but what of the intermediate values? AFAICT this RFC only looks at wakeups, but I guess latency-nice can be applied elsewhere (e.g. load-balance, something like task_hot() and its use of sysctl_sched_migration_cost). > Moreover, we will never match completely the nice semantic, give that > a 1 nice unit has a proper math meaning, isn't something like 10% CPU > usage change for each step? > > For latency-nice instead we will likely base our biasing strategies on > some predefined (maybe system-wide configurable) const thresholds. > > Could changing the name to "latency-tolerance" break the tie by marking > its difference wrt prior/nice levels? AFAIR, that was also the original > proposal [1] by PaulT during the OSPM discussion. > > Best, > Patrick > > [1] https://youtu.be/oz43thSFqmk?t=1302 >