Re: default cpufreq gov, was: [PATCH] sched/fair: check for idle core

Mel Gorman
2020-10-22T16:29:52Z

On Thu, Oct 22, 2020 at 05:25:14PM +0200, Peter Zijlstra wrote:
> On Thu, Oct 22, 2020 at 03:52:50PM +0100, Mel Gorman wrote:
>
> > There are some questions
> > currently on whether schedutil is good enough when HWP is not available.
>
> Srinivas and Rafael will know better, but Intel does run a lot of tests
> and IIRC it was found that schedutil was on-par for !HWP. That was the
> basis for commit:
>
> 33aa46f252c7 ("cpufreq: intel_pstate: Use passive mode by default without HWP")
>
> But now it turns out that commit results in running intel_pstate-passive
> on ondemand, which is quite horrible.
>

I know Intel ran a lot of tests, no question about it and no fingers are
being pointed. I know I've had enough bugs patches tested with a battery
of tests on various machines and still ended up with bug reports :)

> > There was some evidence (I don't have the data, Giovanni was looking into
> > it) that HWP was a requirement to make schedutil work well.
>
> That seems to be the question; Rafael just said the opposite.
>
> > For distros, switching to schedutil by default would be nice because
> > frequency selection state would follow the task instead of being per-cpu
> > and we could stop worrying about different HWP implementations but it's
>
> s/HWP/cpufreq-governors/ ? But yes.
>

I've seen cases where HWP had variable behaviour between CPU
generations. It was hard to quantify and/or figure out because HWP is a
black box.

> > not at the point where the switch is advisable. I would expect hard data
> > before switching the default and still would strongly advise having a
> > period of time where we can fall back when someone inevitably finds a
> > new corner case or exception.
>
> Which is why I advocated to make it 'difficult' to use the old ones and
> only later remove them.
>

That's fair.

--
Mel Gorman
SUSE Labs