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Date: Thu, 9 Apr 2020 16:13:58 +0200 From: Vincent Guittot <vincent.guittot@...aro.org> To: Dietmar Eggemann <dietmar.eggemann@....com> Cc: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>, Juri Lelli <juri.lelli@...hat.com>, Steven Rostedt <rostedt@...dmis.org>, Luca Abeni <luca.abeni@...tannapisa.it>, Daniel Bristot de Oliveira <bristot@...hat.com>, Wei Wang <wvw@...gle.com>, Quentin Perret <qperret@...gle.com>, Alessio Balsini <balsini@...gle.com>, Pavan Kondeti <pkondeti@...eaurora.org>, Patrick Bellasi <patrick.bellasi@...bug.net>, Morten Rasmussen <morten.rasmussen@....com>, Valentin Schneider <valentin.schneider@....com>, Qais Yousef <qais.yousef@....com>, linux-kernel <linux-kernel@...r.kernel.org> Subject: Re: [PATCH 1/4] sched/topology: Store root domain CPU capacity sum On Thu, 9 Apr 2020 at 15:50, Dietmar Eggemann <dietmar.eggemann@....com> wrote: > > On 08.04.20 19:03, Vincent Guittot wrote: > > On Wed, 8 Apr 2020 at 18:31, Dietmar Eggemann <dietmar.eggemann@....com> wrote: > >> > >> On 08.04.20 14:29, Vincent Guittot wrote: > >>> On Wed, 8 Apr 2020 at 11:50, Dietmar Eggemann <dietmar.eggemann@....com> wrote: > >> > >> [...] > >> > >>>> /** > >>>> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c > >>>> index 8344757bba6e..74b0c0fa4b1b 100644 > >>>> --- a/kernel/sched/topology.c > >>>> +++ b/kernel/sched/topology.c > >>>> @@ -2052,12 +2052,17 @@ build_sched_domains(const struct cpumask *cpu_map, struct sched_domain_attr *att > >>>> /* Attach the domains */ > >>>> rcu_read_lock(); > >>>> for_each_cpu(i, cpu_map) { > >>>> + unsigned long cap = arch_scale_cpu_capacity(i); > >>> > >>> Why do you replace the use of rq->cpu_capacity_orig by > >>> arch_scale_cpu_capacity(i) ? > >>> There is nothing about this change in the commit message > >> > >> True. And I can change this back. > >> > >> It seems though that the solution is not sufficient because of the > >> 'rd->span &nsub cpu_active_mask' issue discussed under patch 2/4. > >>ap > >> But this remind me of another question I have. > >> > >> Currently we use arch_scale_cpu_capacity() more often (16 times) than > >> capacity_orig_of()/rq->cpu_capacity_orig . > >> > >> What's hindering us to remove rq->cpu_capacity_orig and the code around > >> it and solely rely on arch_scale_cpu_capacity()? I mean the arch > >> implementation should be fast. > > > > Or we can do the opposite and only use capacity_orig_of()/rq->cpu_capacity_orig. > > > > Is there a case where the max cpu capacity changes over time ? So I > > would prefer to use cpu_capacity_orig which is a field of scheduler > > instead of always calling an external arch specific function > > I see. So far it only changes during startup. > > And it looks like that asym_cpu_capacity_level() [topology.c] would fail > if we would use capacity_orig_of() instead of arch_scale_cpu_capacity(). Yes I agree. See below > > post_init_entity_util_avg() [fair.c] and sugov_get_util() > [cpufreq_schedutil.c] would be temporarily off until > update_cpu_capacity() has updated cpu_rq(cpu)->cpu_capacity_orig. I think that we could even get rid of this update in update_cpu_capacity(). cpu_capacity_orig should be set while building the sched_domain topology because the topology itself is built based on this max cpu capacity with asym_cpu_capacity_level(). So changing the capacity without rebuilding the domain could break the sched_domain topology correctness. And we can't really set cpu_capacity_orig earlier during the boot because the capacity of b.L is set late during the boot and a rebuild of the sched_domain topology is then triggered. > > compute_energy() [fair.c] is guarded by sched_energy_enabled() from > being used at startup. > > scale_rt_capacity() could be changed in case we call it after the > cpu_rq(cpu)->cpu_capacity_orig = arch_scale_cpu_capacity(cpu) in > update_cpu_capacity(). With the removal of the update in update_cpu_capacity(), we don't have a problem anymore, isn't it ? > > The Energy Model (and CPUfreq cooling) code would need > capacity_orig_of() exported. arch_scale_cpu_capacity() currently is > exported via include/linux/sched/topology.h. Not sure that we need to export it outside scheduler, they can still use arch_scale_cpu_capacity() > > I guess Pelt and 'scale invariant Deadline bandwidth enforcement' should > continue using arch_scale_cpu_capacity() in sync with > arch_scale_freq_capacity(). Why can't they use capacity_orig_of ? we keep using arch_scale_freq_capacity() because it's dynamic but we don't really need to keep using arch_scale_cpu_capacity() > > IMHO it's hard to give clear advice when to use the one or the other. > > We probably don't want to set cpu_rq(cpu)->cpu_capacity_orig in the arch > cpu scale setter. We have arch_scale_cpu_capacity() to decouple that. Yes I agree
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