Re: [PATCH v9 05/10] sched: make scale_rt invariant with frequency

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

On 25 November 2014 at 03:24, Wanpeng Li <kernellwp@gmail.com> wrote:
> Hi Vincent,
> On 11/4/14, 12:54 AM, Vincent Guittot wrote:
>>
>> The average running time of RT tasks is used to estimate the remaining
>> compute
>> capacity for CFS tasks. This remaining capacity is the original capacity
>> scaled
>> down by a factor (aka scale_rt_capacity). This estimation of available
>> capacity
>> must also be invariant with frequency scaling.
>>
>> A frequency scaling factor is applied on the running time of the RT tasks
>> for
>> computing scale_rt_capacity.
>>
>> In sched_rt_avg_update, we scale the RT execution time like below:
>> rq->rt_avg += rt_delta * arch_scale_freq_capacity() >>
>> SCHED_CAPACITY_SHIFT
>>
>> Then, scale_rt_capacity can be summarized by:
>> scale_rt_capacity = SCHED_CAPACITY_SCALE -
>>                 ((rq->rt_avg << SCHED_CAPACITY_SHIFT) / period)
>
>
> The 'period' aka 'total' in the scale_rt_capacity(), why it is
> sched_avg_period() + delta instead of sched_avg_period()?

The default value of sched_avg_period is 1sec which is "long" so we
take into account the time consumed by RT tasks in the ongoing period
.

>
> Regards,
> Wanpeng Li
>
>
>>
>> We can optimize by removing right and left shift in the computation of
>> rq->rt_avg
>> and scale_rt_capacity
>>
>> The call to arch_scale_frequency_capacity in the rt scheduling path might
>> be
>> a concern for RT folks because I'm not sure whether we can rely on
>> arch_scale_freq_capacity to be short and efficient ?
>>
>> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
>> ---
>>   kernel/sched/fair.c  | 17 +++++------------
>>   kernel/sched/sched.h |  4 +++-
>>   2 files changed, 8 insertions(+), 13 deletions(-)
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index a5039da..b37c27b 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -5785,7 +5785,7 @@ unsigned long __weak arch_scale_cpu_capacity(struct
>> sched_domain *sd, int cpu)
>>   static unsigned long scale_rt_capacity(int cpu)
>>   {
>>         struct rq *rq = cpu_rq(cpu);
>> -       u64 total, available, age_stamp, avg;
>> +       u64 total, used, age_stamp, avg;
>>         s64 delta;
>>         /*
>> @@ -5801,19 +5801,12 @@ static unsigned long scale_rt_capacity(int cpu)
>>         total = sched_avg_period() + delta;
>>   -     if (unlikely(total < avg)) {
>> -               /* Ensures that capacity won't end up being negative */
>> -               available = 0;
>> -       } else {
>> -               available = total - avg;
>> -       }
>> +       used = div_u64(avg, total);
>>   -     if (unlikely((s64)total < SCHED_CAPACITY_SCALE))
>> -               total = SCHED_CAPACITY_SCALE;
>> +       if (likely(used < SCHED_CAPACITY_SCALE))
>> +               return SCHED_CAPACITY_SCALE - used;
>>   -     total >>= SCHED_CAPACITY_SHIFT;
>> -
>> -       return div_u64(available, total);
>> +       return 1;
>>   }
>>     static void update_cpu_capacity(struct sched_domain *sd, int cpu)
>> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
>> index c34bd11..fc5b152 100644
>> --- a/kernel/sched/sched.h
>> +++ b/kernel/sched/sched.h
>> @@ -1312,9 +1312,11 @@ static inline int hrtick_enabled(struct rq *rq)
>>     #ifdef CONFIG_SMP
>>   extern void sched_avg_update(struct rq *rq);
>> +extern unsigned long arch_scale_freq_capacity(struct sched_domain *sd,
>> int cpu);
>> +
>>   static inline void sched_rt_avg_update(struct rq *rq, u64 rt_delta)
>>   {
>> -       rq->rt_avg += rt_delta;
>> +       rq->rt_avg += rt_delta * arch_scale_freq_capacity(NULL,
>> cpu_of(rq));
>>         sched_avg_update(rq);
>>   }
>>   #else
>
>