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Date: Tue, 4 Sep 2018 11:59:08 +0100 From: Quentin Perret <quentin.perret@....com> To: peterz@...radead.org, rjw@...ysocki.net, linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org Cc: gregkh@...uxfoundation.org, mingo@...hat.com, dietmar.eggemann@....com, morten.rasmussen@....com, chris.redpath@....com, patrick.bellasi@....com, valentin.schneider@....com, vincent.guittot@...aro.org, thara.gopinath@...aro.org, viresh.kumar@...aro.org, tkjos@...gle.com, joel@...lfernandes.org, smuckle@...gle.com, adharmap@...eaurora.org, skannan@...eaurora.org, pkondeti@...eaurora.org, juri.lelli@...hat.com, edubezval@...il.com, srinivas.pandruvada@...ux.intel.com, currojerez@...eup.net, javi.merino@...nel.org Subject: Re: [PATCH v6 13/14] sched/topology: Make Energy Aware Scheduling depend on schedutil On Monday 20 Aug 2018 at 10:44:19 (+0100), Quentin Perret wrote: > Energy Aware Scheduling (EAS) is designed with the assumption that > frequencies of CPUs follow their utilization value. When using a CPUFreq > governor other than schedutil, the chances of this assumption being true > are small, if any. When schedutil is being used, EAS' predictions are at > least consistent with the frequency requests. Although those requests > have no guarantees to be honored by the hardware, they should at least > guide DVFS in the right direction and provide some hope in regards to the > EAS model being accurate. > > To make sure EAS is only used in a sane configuration, create a strong > dependency on schedutil being used. Since having sugov compiled-in does > not provide that guarantee, extend the existing CPUFreq policy notifier > with a new case on governor changes. That allows the scheduler to > register a callback on this notifier to rebuild the scheduling domains > when governors are changed, and enable/disable EAS accordingly. > > cc: Ingo Molnar <mingo@...hat.com> > cc: Peter Zijlstra <peterz@...radead.org> > Signed-off-by: Quentin Perret <quentin.perret@....com> > > --- > This patch could probably be squashed into another one, but I kept it > separate to ease the review. Also, it's probably optional as not having > it will not 'break' things per se. > > I went for the smallest possible solution I could find, which has the > good side of being simple, but it's definitely not the only one. > > Another possibility would be to hook things in sugov_start() and > sugov_stop(), but that requires some more work. In this case, it > wouldn't be possible to just re-build the sched_domains() from there, > because when sugov_stop() is called, the 'governor' field of the policy > hasn't been updated yet, so the condition (if gov == schedutil) in > build_freq_domains() doesn't work. > > To workaround the issue we'll need to find a way to pass a cpumask to > the topology code to specifically say 'sugov has been stopped on these > CPUs'. That would mean more code to handle that, but that would also > mean we don't have to mess around with the CPUFreq notifiers ... > > Not sure what's best, so all feedback is more than welcome. Hi, Does anybody have concerns with this patch ? Is it a reasonable option to use the CPUFreq notifiers in this case ? If there is anything I can do to ease the review please let me know. Also, is there any hope that the 12 first patches could make it in 4.20 on their own ? Or is it already too late ? Thanks, Quentin
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