Re: [PATCH v5 00/10] track CPU utilization

[Vincent Guittot <vincent.guittot@linaro.org>]

On 6 June 2018 at 11:44, Quentin Perret <quentin.perret@arm.com> wrote:
> On Tuesday 05 Jun 2018 at 16:18:09 (+0200), Peter Zijlstra wrote:
>> On Mon, Jun 04, 2018 at 08:08:58PM +0200, Vincent Guittot wrote:
>> > On 4 June 2018 at 18:50, Peter Zijlstra <peterz@infradead.org> wrote:
>>
>> > > So this patch-set tracks the !cfs occupation using the same function,
>> > > which is all good. But what, if instead of using that to compensate the
>> > > OPP selection, we employ that to renormalize the util signal?
>> > >
>> > > If we normalize util against the dynamic (rt_avg affected) cpu_capacity,
>> > > then I think your initial problem goes away. Because while the RT task
>> > > will push the util to .5, it will at the same time push the CPU capacity
>> > > to .5, and renormalized that gives 1.
>> > >
>> > >   NOTE: the renorm would then become something like:
>> > >         scale_cpu = arch_scale_cpu_capacity() / rt_frac();
>>
>> Should probably be:
>>
>>       scale_cpu = atch_scale_cpu_capacity() / (1 - rt_frac())
>>
>> > >
>> > >
>> > > On IRC I mentioned stopping the CFS clock when preempted, and while that
>> > > would result in fixed numbers, Vincent was right in pointing out the
>> > > numbers will be difficult to interpret, since the meaning will be purely
>> > > CPU local and I'm not sure you can actually fix it again with
>> > > normalization.
>> > >
>> > > Imagine, running a .3 RT task, that would push the (always running) CFS
>> > > down to .7, but because we discard all !cfs time, it actually has 1. If
>> > > we try and normalize that we'll end up with ~1.43, which is of course
>> > > completely broken.
>> > >
>> > >
>> > > _However_, all that happens for util, also happens for load. So the above
>> > > scenario will also make the CPU appear less loaded than it actually is.
>> >
>> > The load will continue to increase because we track runnable state and
>> > not running for the load
>>
>> Duh yes. So renormalizing it once, like proposed for util would actually
>> do the right thing there too.  Would not that allow us to get rid of
>> much of the capacity magic in the load balance code?
>>
>> /me thinks more..
>>
>> Bah, no.. because you don't want this dynamic renormalization part of
>> the sums. So you want to keep it after the fact. :/
>>
>> > As you mentioned, scale_rt_capacity give the remaining capacity for
>> > cfs and it will behave like cfs util_avg now that it uses PELT. So as
>> > long as cfs util_avg <  scale_rt_capacity(we probably need a margin)
>> > we keep using dl bandwidth + cfs util_avg + rt util_avg for selecting
>> > OPP because we have remaining spare capacity but if  cfs util_avg ==
>> > scale_rt_capacity, we make sure to use max OPP.
>>
>> Good point, when cfs-util < cfs-cap then there is idle time and the util
>> number is 'right', when cfs-util == cfs-cap we're overcommitted and
>> should go max.
>>
>> Since the util and cap values are aligned that should track nicely.
>
> So Vincent proposed to have a margin between cfs util and cfs cap to be
> sure there is a little bit of idle time. This is _exactly_ what the
> overutilized flag in EAS does. That would actually make a lot of sense
> to use that flag in schedutil. The idea is basically to say, if there
> isn't enough idle time on all CPUs, the util signal are kinda wrong, so
> let's not make any decisions (task placement or OPP selection) based on
> that. If overutilized, go to max freq. Does that make sense ?

Yes it's similar to the overutilized except that
- this is done per cpu and whereas overutilization is for the whole system
- the test is done at every freq update and not only during some cfs
event and it uses the last up to date value and not a periodically
updated snapshot of the value
- this is done also without EAS

Then for the margin, it has to be discussed if it is really needed or not

>
> Thanks,
> Quentin