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

[Vincent Guittot]

On 15 December 2015 at 09:50, Luca Abeni <luca.abeni@xxxxxxxx> wrote:
> On 12/15/2015 05:59 AM, Vincent Guittot wrote:
> [...]
>>>>>
>>>>> 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.
>>>>
>>>>

[snip]

>> The 2nd definition is used to compute the remaining capacity for the
>> CFS scheduler. This one doesn't need to be updated at each wake/sleep
>> of a deadline task but should reflect the capacity used by deadline in
>> a larger time scale. The latter will be used by the CFS scheduler  at
>> the periodic load balance pace
>
> Ok, so as I wrote above this really looks like an average utilisation.
> My impression (but I do not know the CFS code too much) is that the mainline
> kernel is currently doing the right thing to compute it, so maybe there is
> no
> need to change the current code in this regard.
> If the current code is not acceptable for some reason, an alternative would
> be to measure the active utilisation for frequency scaling, and then apply a
> low-pass filter to it for CFS.

In this case, it's probably easier to split what is already done into
a rt_avg metric  and a dl_avg metric

Vincent
>
>
>                                 Luca
>
>
>>
>>> As done, for example, here:
>>> https://github.com/lucabe72/linux-reclaiming/tree/track-utilisation-v2
>>> (in particular, see
>>>
>>> https://github.com/lucabe72/linux-reclaiming/commit/49fc786a1c453148625f064fa38ea538470df55b
>>> )
>>> I understand this approach might look too complex... But I think it is
>>> much less pessimistic while still being "safe".
>>> If there is something that I can do to make that code more acceptable,
>>> let me know.
>>>
>>>
>>>                          Luca
>
>
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