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

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

On 24 November 2014 at 18:05, Morten Rasmussen <morten.rasmussen@arm.com> wrote:
> On Mon, Nov 24, 2014 at 02:24:00PM +0000, Vincent Guittot wrote:
>> On 21 November 2014 at 13:35, Morten Rasmussen <morten.rasmussen@arm.com> wrote:
>> > On Mon, Nov 03, 2014 at 04:54:42PM +0000, Vincent Guittot wrote:
>>
>> [snip]
>>
>> >> The average running time of RT tasks is used to estimate the remaining compute
>> >> @@ -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);
>> >
>> > I haven't looked through all the details of the rt avg tracking, but if
>> > 'used' is in the range [0..SCHED_CAPACITY_SCALE], I believe it should
>> > work. Is it guaranteed that total > 0 so we don't get division by zero?
>>
>> static inline u64 sched_avg_period(void)
>> {
>> return (u64)sysctl_sched_time_avg * NSEC_PER_MSEC / 2;
>> }
>>
>
> I see.
>
>> >
>> > It does get a slightly more complicated if we want to figure out the
>> > available capacity at the current frequency (current < max) later. Say,
>> > rt eats 25% of the compute capacity, but the current frequency is only
>> > 50%. In that case get:
>> >
>> > curr_avail_capacity = (arch_scale_cpu_capacity() *
>> >   (arch_scale_freq_capacity() - (SCHED_SCALE_CAPACITY - scale_rt_capacity())))
>> >   >> SCHED_CAPACITY_SHIFT
>>
>> You don't have to be so complicated but simply need to do:
>> curr_avail_capacity for CFS = (capacity_of(CPU) *
>> arch_scale_freq_capacity())  >> SCHED_CAPACITY_SHIFT
>>
>> capacity_of(CPU) = 600 is the max available capacity for CFS tasks
>> once we have removed the 25% of capacity that is used by RT tasks
>> arch_scale_freq_capacity = 512 because we currently run at 50% of max freq
>>
>> so curr_avail_capacity for CFS = 300
>
> I don't think that is correct. It is at least not what I had in mind.
>
> capacity_orig_of(cpu) = 800, we run at 50% frequency which means:
>
> curr_capacity = capacity_orig_of(cpu) * arch_scale_freq_capacity()
>                   >> SCHED_CAPACITY_SHIFT
>               = 400
>
> So the total capacity at the current frequency (50%) is 400, without
> considering RT. scale_rt_capacity() is frequency invariant, so it takes
> away capacity_orig_of(cpu) - capacity_of(cpu) = 200 worth of capacity
> for RT.  We need to subtract that from the current capacity to get the
> available capacity at the current frequency.
>
> curr_available_capacity = curr_capacity - (capacity_orig_of(cpu) -
> capacity_of(cpu)) = 200

you're right, this one looks good to me too

>
> In other words, 800 is the max capacity, we are currently running at 50%
> frequency, which gives us 400. RT takes away 25% of 800
> (frequency-invariant) from the 400, which leaves us with 200 left for
> CFS tasks at the current frequency.
>
> In your calculations you subtract the RT load before computing the
> current capacity using arch_scale_freq_capacity(), where I think it
> should be done after. You find the amount spare capacity you would have
> at the maximum frequency when RT has been subtracted and then scale the
> result by frequency which means indirectly scaling the RT load
> contribution again (the rt avg has already been scaled). So instead of
> taking away 200 of the 400 (current capacity @ 50% frequency), it only
> takes away 100 which isn't right.
>
> scale_rt_capacity() is frequency-invariant, so if the RT load is 50% and
> the frequency is 50%, there are no spare cycles left.
> curr_avail_capacity should be 0. But using your expression above you
> would get capacity_of(cpu) = 400 after removing RT,
> arch_scale_freq_capacity = 512 and you get 200. I don't think that is
> right.
>
> Morten
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