Re: [PATCH v2 1/2] sched/schedutil: rework performance estimation

[Lukasz Luba]

Hi Vincent,

I know that there is v3, but just to respond to this below.

On 10/31/23 09:48, Vincent Guittot wrote:
> Hi Lukasz,
>
> On Mon, 30 Oct 2023 at 18:45, Lukasz Luba <lukasz.luba@xxxxxxx> wrote:
> > Hi Vincent,
> >
> > On 10/26/23 18:09, Vincent Guittot wrote:
> > > The current method to take into account uclamp hints when estimating the
> > > target frequency can end into situation where the selected target
> > > frequency is finally higher than uclamp hints whereas there are no real
> > > needs. Such cases mainly happen because we are currently mixing the
> > > traditional scheduler utilization signal with the uclamp performance
> > > hints. By adding these 2 metrics, we loose an important information when
> > > it comes to select the target frequency and we have to make some
> > > assumptions which can't fit all cases.
> > >
> > > Rework the interface between the scheduler and schedutil governor in order
> > > to propagate all information down to the cpufreq governor.
> > >
> > > effective_cpu_util() interface changes and now returns the actual
> > > utilization of the CPU with 2 optional inputs:
> > > - The minimum performance for this CPU; typically the capacity to handle
> > >     the deadline task and the interrupt pressure. But also uclamp_min
> > >     request when available.
> > > - The maximum targeting performance for this CPU which reflects the
> > >     maximum level that we would like to not exceed. By default it will be
> > >     the CPU capacity but can be reduced because of some performance hints
> > >     set with uclamp. The value can be lower than actual utilization and/or
> > >     min performance level.
> >
> > You have probably missed my question in the last v1 patch set.
>
> Yes, sorry
>
> > The description above needs a bit of clarification, since looking at the
> > patches some dark corners are introduced IMO:
> >
> > Currently, we have a less aggressive power saving policy than this
> > proposal.
> >
> > The questions:
> > What if the PD has 4 CPUs, the max util found is 500 and is from a CPU
> > w/ uclamp_max, but there is another CPU with normal utilization 499?
> > What should be the final frequency for that PD?
>
> We now follow the same sequence everywhere which can be summarized by:
>
> for each cpu sharing the same frequency domain:
>      util = cpu_util(cpu)
>      eff_util = effective_cpu_util(util, &min, &max)
>      eff_util = sugov_effective_cpu_perf(eff_util, min, max) which
> applies the dvfs headroom if needed
>      max_util = max(max_util, eff_util);
>
> EAS anticipates the impact of the waking task on utilization and max
> but the end result is the same as above once the task is enqueued so I
> didn't show it for simplicity
>
> Coming back to your example
>    CPU0 has uclamp_max = 500 and an actual utilization above 500. Its
> eff_util will be 500
>    CPU1 doesn't have uclamp_max constraint and an actual utilization of
> 499 which will be increase with dvfs headroom to 623 in
> sugov_effective_cpu_perf()
>
> The final max util will be 623
>
> With the current implementation we apply the dvfs headroom to the
> final max_util (which is the CPU0 with uclamp_max == 500) whereas we
> now apply the dvfs headroom on each CPU inside
> sugov_effective_cpu_perf()
>
> The main difference is that if CPU1 has an actual utilization of 400,
> the max_util of the frequency domain will be 500 whereas it is 625
> after applying dvfs headroom with current implementation
>
> > In current design, where we care more about 'delivered performance
> > to the tasks' than power saving, the +20% would be applied for the
> > frequency. Therefore if that CPU with 499 util doesn't have uclamp_max,
> > it would get a decent amount of idle time for its tasks (to compensate
> > some workload variation).
>
> CPU1 with 499 still gets its 25% margin or I missed something in your example ?

You understood this correctly. I don't have more questions. It should
than work OK.

Thanks,
Lukasz