Re: [PATCH v6 13/14] sched/topology: Make Energy Aware Scheduling depend on schedutil

[Quentin Perret <quentin.perret@arm.com>]

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@redhat.com>
> cc: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Quentin Perret <quentin.perret@arm.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