From mboxrd@z Thu Jan 1 00:00:00 1970 From: Quentin Perret Subject: Re: [PATCH] Revert "cpufreq: schedutil: Don't restrict kthread to related_cpus unnecessarily" Date: Tue, 8 May 2018 10:42:37 +0100 Message-ID: <20180508094237.GA3752@e108498-lin.cambridge.arm.com> References: <20180508073340.13114-1-dietmar.eggemann@arm.com> <20180508082242.bre6sjfvefhz6xc3@vireshk-i7> <8cf21b1a-ca6e-fed7-43c5-94c66ff5986b@arm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <8cf21b1a-ca6e-fed7-43c5-94c66ff5986b@arm.com> Sender: linux-kernel-owner@vger.kernel.org To: Dietmar Eggemann Cc: Viresh Kumar , linux-kernel@vger.kernel.org, Peter Zijlstra , Ingo Molnar , linux-pm@vger.kernel.org, Pavan Kondeti , "Rafael J . Wysocki" , Juri Lelli , Joel Fernandes , Patrick Bellasi List-Id: linux-pm@vger.kernel.org On Tuesday 08 May 2018 at 11:09:57 (+0200), Dietmar Eggemann wrote: > On 05/08/2018 10:22 AM, Viresh Kumar wrote: > > On 08-05-18, 08:33, Dietmar Eggemann wrote: > > > This reverts commit e2cabe48c20efb174ce0c01190f8b9c5f3ea1d13. > > > > > > Lifting the restriction that the sugov kthread is bound to the > > > policy->related_cpus for a system with a slow switching cpufreq driver, > > > which is able to perform DVFS from any cpu (e.g. cpufreq-dt), is not > > > only not beneficial it also harms Enery-Aware Scheduling (EAS) on > > > systems with asymmetric cpu capacities (e.g. Arm big.LITTLE). > > > > > > The sugov kthread which does the update for the little cpus could > > > potentially run on a big cpu. It could prevent that the big cluster goes > > > into deeper idle states although all the tasks are running on the little > > > cluster. > > > > I think the original patch did the right thing, but that doesn't suit > > everybody as you explained. > > > > I wouldn't really revert the patch but fix my platform's cpufreq > > driver to set dvfs_possible_from_any_cpu = false, so that other > > platforms can still benefit from the original commit. > > This would make sure that the kthreads are bound to the correct set of cpus > for platforms with those cpufreq drivers (cpufreq-dt (h960), scmi-cpufreq, > scpi-cpufreq) but it will also change the logic (e.g. > sugov_should_update_freq() -> cpufreq_can_do_remote_dvfs()). > > I'm still struggling to understand when a driver/platform should set > dvfs_possible_from_any_cpu to true and what the actual benefit would be. I assume it might be beneficial to have the kthread moving around freely in some cases, but since it is a SCHED_DEADLINE task now it can't really migrate anywhere anyway. So I'm not sure either if this commits still makes sense now. Or is there another use case for this ? Thanks, Quentin