Re: [RFC PATCH 0/7] sched: cpufreq: Remove magic margins

Lukasz Luba
2023-09-07T15:53:39Z

On 9/7/23 15:29, Peter Zijlstra wrote:
> On Thu, Sep 07, 2023 at 02:57:26PM +0100, Lukasz Luba wrote:
>>
>>
>> On 9/7/23 14:26, Peter Zijlstra wrote:
>>> On Wed, Sep 06, 2023 at 10:18:50PM +0100, Qais Yousef wrote:
>>>
>>>> This is probably controversial statement. But I am not in favour of util_est.
>>>> I need to collect the data, but I think we're better with 16ms PELT HALFLIFE as
>>>> default instead. But I will need to do a separate investigation on that.
>>>
>>> I think util_est makes perfect sense, where PELT has to fundamentally
>>> decay non-running / non-runnable tasks in order to provide a temporal
>>> average, DVFS might be best served with a termporal max filter.
>>>
>>>
>>
>> Since we are here...
>> Would you allow to have a configuration for
>> the util_est shifter: UTIL_EST_WEIGHT_SHIFT ?
>>
>> I've found other values than '2' better in some scenarios. That helps
>> to prevent a big task to 'down' migrate from a Big CPU (1024) to some
>> Mid CPU (~500-700 capacity) or even Little (~120-300).
>
> Larger values, I'm thinking you're after? Those would cause the new
> contribution to weight less, making the function more smooth, right?

Yes, more smooth, because we only use the 'ewma' goodness for decaying
part (not the raising [1]).

>
> What task characteristic is tied to this? That is, this seems trivial to
> modify per-task.

In particular Speedometer test and the main browser task, which reaches
~900util, but sometimes vanish and waits for other background tasks
to do something. In the meantime it can decay and wake-up on
Mid/Little (which can cause a penalty to score up to 5-10% vs. if
we pin the task to big CPUs). So, a longer util_est helps to avoid
at least very bad down migration to Littles...

[1] https://elixir.bootlin.com/linux/v6.5.1/source/kernel/sched/fair.c#L4442