Re: [RFCv6 PATCH 09/10] sched: deadline: use deadline bandwidth in scale_rt_capacity

[Vincent Guittot]

On 14 December 2015 at 16:17, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> On Tue, Dec 08, 2015 at 10:19:30PM -0800, Steve Muckle wrote:
>> From: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
>
>> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
>> index 8b0a15e..9d9eb50 100644
>> --- a/kernel/sched/deadline.c
>> +++ b/kernel/sched/deadline.c
>> @@ -43,6 +43,24 @@ static inline int on_dl_rq(struct sched_dl_entity *dl_se)
>>       return !RB_EMPTY_NODE(&dl_se->rb_node);
>>  }
>>
>> +static void add_average_bw(struct sched_dl_entity *dl_se, struct dl_rq *dl_rq)
>> +{
>> +     u64 se_bw = dl_se->dl_bw;
>> +
>> +     dl_rq->avg_bw += se_bw;
>> +}
>> +
>> +static void clear_average_bw(struct sched_dl_entity *dl_se, struct dl_rq *dl_rq)
>> +{
>> +     u64 se_bw = dl_se->dl_bw;
>> +
>> +     dl_rq->avg_bw -= se_bw;
>> +     if (dl_rq->avg_bw < 0) {
>> +             WARN_ON(1);
>> +             dl_rq->avg_bw = 0;
>> +     }
>> +}
>
>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index 4c49f76..ce05f61 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -6203,6 +6203,14 @@ static unsigned long scale_rt_capacity(int cpu)
>>
>>       used = div_u64(avg, total);
>>
>> +     /*
>> +      * deadline bandwidth is defined at system level so we must
>> +      * weight this bandwidth with the max capacity of the system.
>> +      * As a reminder, avg_bw is 20bits width and
>> +      * scale_cpu_capacity is 10 bits width
>> +      */
>> +     used += div_u64(rq->dl.avg_bw, arch_scale_cpu_capacity(NULL, cpu));
>> +
>>       if (likely(used < SCHED_CAPACITY_SCALE))
>>               return SCHED_CAPACITY_SCALE - used;
>>
>> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
>> index 08858d1..e44c6be 100644
>> --- a/kernel/sched/sched.h
>> +++ b/kernel/sched/sched.h
>> @@ -519,6 +519,8 @@ struct dl_rq {
>>  #else
>>       struct dl_bw dl_bw;
>>  #endif
>> +     /* This is the "average utilization" for this runqueue */
>> +     s64 avg_bw;
>>  };
>
> So I don't think this is right. AFAICT this projects the WCET as the
> amount of time actually used by DL. This will, under many circumstances,
> vastly overestimate the amount of time actually spend on it. Therefore
> unduly pessimisme the fair capacity of this CPU.

I agree that if the WCET is far from reality, we will underestimate
available capacity for CFS. Have you got some use case in mind which
overestimates the WCET ?
If we can't rely on this parameters to evaluate the amount of capacity
used by deadline scheduler on a core, this will imply that we can't
also use it for requesting capacity to cpufreq and we should fallback
on a monitoring mechanism which reacts to a change instead of
anticipating it.

>
>
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