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

[Luca Abeni <luca.abeni@unitn.it>]

On 12/15/2015 01:43 PM, Vincent Guittot wrote:
[...]
>>>>> 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.
>>>>
>>>> I think a more "theoretically sound" approach would be to track the
>>>> _active_ utilisation (informally speaking, the sum of the utilisations
>>>> of the tasks that are actually active on a core - the exact definition
>>>> of "active" is the trick here).
>>>
>>>
>>> The point is that we probably need 2 definitions of "active" tasks.
>>
>> Ok; thanks for clarifying. I do not know much about the remaining capacity
>> used by CFS; however, from what you write I guess CFS really need an
>> "average"
>> utilisation (while frequency scaling needs the active utilisation).
>
> yes. this patch is only about the "average" utilization
Ok; so, I think that the approach of this patch is too pessimistic (it uses
the "worst-case" utilisation as an estimation of the average one).

>>> This one
>>> should be updated quite often with the wake up and the sleep of tasks
>>> as well as the throttling.
>>
>> Strictly speaking, the active utilisation must be updated when a task
>> wakes up and when a task sleeps/terminates (but when a task
>> sleeps/terminates
>> you cannot decrease the active utilisation immediately: you have to wait
>> some time because the task might already have used part of its "future
>> utilisation").
>> The active utilisation must not be updated when a task is throttled: a
>> task is throttled when its current runtime is 0, so it already used all
>> of its utilisation for the current period (think about two tasks with
>> runtime=50ms and period 100ms: they consume 100% of the time on a CPU,
>> and when the first task consumed all of its runtime, you cannot decrease
>> the active utilisation).
>
>   I haven't read the paper you pointed in the previous email but it's
> on my todo list. Does the GRUB-PA take into account the frequency
> transition when selecting the best frequency ?
I do not know...
As far as I understand, the GRUB-PA approach is simple: if the active
utilisation of SCHED_DEADLINE tasks is Ua, then the CPU frequency can be
reduced to the maximum possible frequency multiplied by Ua (of course,
this must be adjusted a little bit, because the original GRUB-PA paper
only considered real-time/SCHED_DEADLINE tasks... To leave some CPU time
for other tasks, you have to increase Ua a little bit).

Some time ago one of the authors of the GRUB-PA paper told me that they
evaluated the performance of the algorithm by simulating the behaviour of
a real CPU, but I do not know the details, and I do not know if they took
the frequency transition into account.



				Luca