Re: [PATCH v3 09/12] Revert "sched: Put rq's sched_avg under CONFIG_FAIR_GROUP_SCHED"

[Vincent Guittot]

On 11 July 2014 17:13, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> On Fri, Jul 11, 2014 at 09:51:06AM +0200, Vincent Guittot wrote:
>> On 10 July 2014 15:16, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>> > On Mon, Jun 30, 2014 at 06:05:40PM +0200, Vincent Guittot wrote:
>> >> This reverts commit f5f9739d7a0ccbdcf913a0b3604b134129d14f7e.
>> >>
>> >> We are going to use runnable_avg_sum and runnable_avg_period in order to get
>> >> the utilization of the CPU. This statistic includes all tasks that run the CPU
>> >> and not only CFS tasks.
>> >
>> > But this rq->avg is not the one that is migration aware, right? So why
>> > use this?
>>
>> Yes, it's not the one that is migration aware
>>
>> >
>> > We already compensate cpu_capacity for !fair tasks, so I don't see why
>> > we can't use the migration aware one (and kill this one as Yuyang keeps
>> > proposing) and compensate with the capacity factor.
>>
>> The 1st point is that cpu_capacity is compensated by both !fair_tasks
>> and frequency scaling and we should not take into account frequency
>> scaling for detecting overload
>
> dvfs could help? Also we should not use arch_scale_freq_capacity() for
> things like cpufreq-ondemand etc. Because for those the compute capacity
> is still the max. We should only use it when we hard limit things.

In my mind, arch_scale_cpu_freq was intend to scale the capacity of
the CPU according to the current dvfs operating point.
As it's no more use anywhere now that we have arch_scale_cpu, we could
probably remove it .. and see when it will become used.

>
>> What we have now is the the weighted load avg that is the sum of the
>> weight load of entities on the run queue. This is not usable to detect
>> overload because of the weight. An unweighted version of this figure
>> would be more usefull but it's not as accurate as the one I use IMHO.
>> The example that has been discussed during the review of the last
>> version has shown some limitations
>>
>> With the following schedule pattern from Morten's example
>>
>>    | 5 ms | 5 ms | 5 ms | 5 ms | 5 ms | 5 ms | 5 ms | 5 ms | 5 ms |
>> A:   run     rq     run  ----------- sleeping -------------  run
>> B:   rq      run    rq    run   ---- sleeping -------------  rq
>>
>> The scheduler will see the following values:
>> Task A unweighted load value is 47%
>> Task B unweight load is 60%
>> The maximum Sum of unweighted load is 104%
>> rq->avg load is 60%
>>
>> And the real CPU load is 50%
>>
>> So we will have opposite decision depending of the used values: the
>> rq->avg or the Sum of unweighted load
>>
>> The sum of unweighted load has the main advantage of showing
>> immediately what will be the relative impact of adding/removing a
>> task. In the example, we can see that removing task A or B will remove
>> around half the CPU load but it's not so good for giving the current
>> utilization of the CPU
>
> In that same discussion ISTR a suggestion about adding avg_running time,
> as opposed to the current avg_runnable. The sum of avg_running should be
> much more accurate, and still react correctly to migrations.

I haven't look in details but I agree that avg_running would be much
more accurate than avg_runnable and should probably fit the
requirement. Does it means that we could re-add the avg_running (or
something similar) that has disappeared during the review of load avg
tracking patchset ?

>
>
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