From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dietmar Eggemann Subject: Re: [PATCH v3 06/14] sched/cpufreq: uclamp: add utilization clamping for RT tasks Date: Thu, 16 Aug 2018 12:34:05 +0200 Message-ID: <1b72b94c-5411-4b95-01a6-49ac978acbd5@arm.com> References: <20180806163946.28380-1-patrick.bellasi@arm.com> <20180806163946.28380-7-patrick.bellasi@arm.com> <20180807132630.GB3062@localhost.localdomain> <20180809153423.nsoepprhut3dv4u2@darkstar> <20180813101221.GA2605@e110439-lin> <20180813124911.GD2605@e110439-lin> <20180813150112.GE2605@e110439-lin> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20180813150112.GE2605@e110439-lin> Content-Language: en-GB Sender: linux-kernel-owner@vger.kernel.org To: Patrick Bellasi , Vincent Guittot Cc: Juri Lelli , linux-kernel , linux-pm , Ingo Molnar , Peter Zijlstra , Tejun Heo , "Rafael J. Wysocki" , Viresh Kumar , Paul Turner , Morten Rasmussen , Todd Kjos , Joel Fernandes , Steve Muckle , Suren Baghdasaryan List-Id: linux-pm@vger.kernel.org On 08/13/2018 05:01 PM, Patrick Bellasi wrote: > On 13-Aug 16:06, Vincent Guittot wrote: >> On Mon, 13 Aug 2018 at 14:49, Patrick Bellasi wrote: >>> On 13-Aug 14:07, Vincent Guittot wrote: >>>> On Mon, 13 Aug 2018 at 12:12, Patrick Bellasi wrote: > > [...] > >>> Yes I agree that the current behavior is not completely clean... still >>> the question is: do you reckon the problem I depicted above, i.e. RT >>> workloads eclipsing the min_util required by lower priority classes? >> >> As said above, I don't think that there is a problem that is specific >> to cross class scheduling that can't also happen in the same class. >> >> Regarding your example: >> task TA util=40% with uclamp_min 50% >> task TB util=10% with uclamp_min 0% >> >> If TA and TB are cfs, util=50% and it doesn't seem to be a problem >> whereas TB will steal some bandwidth to TA and delay it (and i even >> don't speak about the impact of the nice priority of TB) >> If TA is cfs and TB is rt, Why util=50% is now a problem for TA ? > > You right, in the current implementation, where we _do not_ > distinguish among scheduling classes it's not possible to get a > reasonable implementation of a per sched class clamping. > >>> To a certain extend I see this problem similar to the rt/dl/irq pressure >>> in defining cpu_capacity, isn't it? > > However, I still think that higher priority classes eclipsing the > clamping of lower priority classes can still be a problem. > > In your example above, the main difference between TA and TB being on > the same class or different classes is that in the second case TB > is granted to always preempt TA. We can end up with a non boosted RT > task consuming all the boosted bandwidth required by a CFS task. > > This does not happen, apart maybe for the corner case of really > different nice values, if the tasks are both CFS, since the fair > scheduler will grant some progress for both of them. > > Thus, given the current implementation, I think it makes sense to drop > the UCLAMP_SCHED_CLASS policy and stick with a more clean and > consistent design. I agree with everything said in this thread so far. So in case you skip UCLAMP_SCHED_CLASS [(B) combine the clamped utilizations] in v4, you will only provide [A) clamp the combined utilization]? I assume that we don't have to guard the util clamping for rt tasks behind a disabled by default sched feature because all runnable rt tasks will have util_min = SCHED_CAPACITY_SCALE by default? > I'll then see if it makes sense to add a dedicated patch on top of the > series to add a proper per-class clamp tracking. I assume if you introduce this per-class clamping you will switch to use the UCLAMP_SCHED_CLASS approach?