Re: [RFC][PATCH v3 2/2] cpufreq: schedutil: Avoid reducing frequency of busy CPUs prematurely

[Vincent Guittot]

On 25 March 2017 at 02:14, Sai Gurrappadi <sgurrappadi@xxxxxxxxxx> wrote:
> Hi Rafael,
>
> On 03/21/2017 04:08 PM, Rafael J. Wysocki wrote:
>> From: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
>>
>> The way the schedutil governor uses the PELT metric causes it to
>> underestimate the CPU utilization in some cases.
>>
>> That can be easily demonstrated by running kernel compilation on
>> a Sandy Bridge Intel processor, running turbostat in parallel with
>> it and looking at the values written to the MSR_IA32_PERF_CTL
>> register.  Namely, the expected result would be that when all CPUs
>> were 100% busy, all of them would be requested to run in the maximum
>> P-state, but observation shows that this clearly isn't the case.
>> The CPUs run in the maximum P-state for a while and then are
>> requested to run slower and go back to the maximum P-state after
>> a while again.  That causes the actual frequency of the processor to
>> visibly oscillate below the sustainable maximum in a jittery fashion
>> which clearly is not desirable.
>>
>> That has been attributed to CPU utilization metric updates on task
>> migration that cause the total utilization value for the CPU to be
>> reduced by the utilization of the migrated task.  If that happens,
>> the schedutil governor may see a CPU utilization reduction and will
>> attempt to reduce the CPU frequency accordingly right away.  That
>> may be premature, though, for example if the system is generally
>> busy and there are other runnable tasks waiting to be run on that
>> CPU already.
>>
>
> Thinking out loud a bit, I wonder if what you really want to do is basically:
>
> schedutil_cpu_util(cpu) = max(cpu_rq(cpu)->cfs.util_avg, total_cpu_util_avg);
>
> Where total_cpu_util_avg tracks the average utilization of the CPU itself over time (% of time the CPU was busy) in the same PELT like manner. The difference here is that it doesn't change instantaneously as tasks migrate in/out but it decays/accumulates just like the per-entity util_avgs.

But we loose the interest of immediate decrease when tasks migrate.
Instead of total_cpu_util_avg we should better track RT utilization in
the same manner so with ongoing work for deadline we will have :
total_utilization = cfs.util_avg + rt's util_avg + deadline's util avg
and we still take advantage of task migration effect


>
> Over time, total_cpu_util_avg and cfs_rq(cpu)->util_avg will tend towards each other the lesser the amount of 'overlap' / overloading.
>
> Yes, the above metric would 'overestimate' in case all tasks have migrated away and we are left with an idle CPU. A fix for that could be to just use the PELT value like so:
>
> schedutil_cpu_util(cpu) = max(cpu_rq(cpu)->cfs.util_avg, idle_cpu(cpu) ? 0 : total_cpu_util_avg);
>
> Note that the problem described here in the commit message doesn't need fully runnable threads, it just needs two threads to execute in parallel on the same CPU for a period of time. I don't think looking at just idle_calls necessarily covers all cases.
>
> Thoughts?
>
> Thanks,
> -Sai