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

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

On 26 November 2014 at 06:18, Wanpeng Li <kernellwp@gmail.com> wrote:
> Hi Vincent,
>
> On 11/25/14, 9:52 PM, Vincent Guittot wrote:
>>
>> 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
>> .
>
>
> Do you mean 'sched_avg_period() + delta' should be replaced by 'delta' since
> sched_avg_period() is "long"?

No, Both are useful. sched_avg_period is useful to keep the value more
stable across the time. Without sched_avg_period,  it will be too much
reactive  ( especially at the beg of a period ).

>
> Regards,
> Wanpeng Li
>
>
>>
>>> 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
>>>
>>>
>