Re: [RFC PATCH 2/2] cpufreq/schedutil: Remove iowait boost

[Christian Loehle <christian.loehle@arm.com>]

On 25/03/2024 02:37, Qais Yousef wrote:
> On 03/18/24 18:08, Rafael J. Wysocki wrote:
>> On Mon, Mar 18, 2024 at 5:40 PM Christian Loehle
>> <christian.loehle@arm.com> wrote:
>>>
>>> On 18/03/2024 14:07, Rafael J. Wysocki wrote:
>>>> On Mon, Mar 4, 2024 at 9:17 PM Christian Loehle
>>>> <christian.loehle@arm.com> wrote:
>>>>>
>>>>> The previous commit provides a new cpu_util_cfs_boost_io interface for
>>>>> schedutil which uses the io boosted utilization of the per-task
>>>>> tracking strategy. Schedutil iowait boosting is therefore no longer
>>>>> necessary so remove it.
>>>>
>>>> I'm wondering about the cases when schedutil is used without EAS.
>>>>
>>>> Are they still going to be handled as before after this change?
>>>
>>> Well they should still get boosted (under the new conditions) and according
>>> to my tests that does work.
>>
>> OK
>>
>>> Anything in particular you're worried about?
>>
>> It is not particularly clear to me how exactly the boost is taken into
>> account without EAS.
>>
>>> So in terms of throughput I see similar results with EAS and CAS+sugov.
>>> I'm happy including numbers in the cover letter for future versions, too.
>>> So far my intuition was that nobody would care enough to include them
>>> (as long as it generally still works).
>>
>> Well, IMV clear understanding of the changes is more important.
> 
> I think the major thing we need to be careful about is the behavior when the
> task is sleeping. I think the boosting will be removed when the task is
> dequeued and I can bet there will be systems out there where the BLOCK softirq
> being boosted when the task is sleeping will matter.

Currently I see this mainly protected by the sugov rate_limit_us.
With the enqueue's being the dominating cpufreq updates it's not really an
issue, the boost is expected to survive the sleep duration, during which it
wouldn't be active.
I did experiment with some sort of 'stickiness' of the boost to the rq, but
it is somewhat of a pain to deal with if we want to remove it once enqueued
on a different rq. A sugov 1ms timer is much simpler of course.
Currently it's not necessary IMO, but for the sake of being future-proof in
terms of more frequent freq updates I might include it in v2.

> 
> FWIW I do have an implementation for per-task iowait boost where I went a step
> further and converted intel_pstate too and like Christian didn't notice
> a regression. But I am not sure (rather don't think) I triggered this use case.
> I can't tell when the systems truly have per-cpu cpufreq control or just appear
> so and they are actually shared but not visible at linux level.

Please do share your intel_pstate proposal!

Kind Regards,
Christian