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

Phil Auld
2020-10-22T15:12:20Z

On Thu, Oct 22, 2020 at 03:58:13PM +0100 Colin Ian King wrote:
> On 22/10/2020 15:52, Mel Gorman wrote:
> > On Thu, Oct 22, 2020 at 02:29:49PM +0200, Peter Zijlstra wrote:
> >> On Thu, Oct 22, 2020 at 02:19:29PM +0200, Rafael J. Wysocki wrote:
> >>>> However I do want to retire ondemand, conservative and also very much
> >>>> intel_pstate/active mode.
> >>>
> >>> I agree in general, but IMO it would not be prudent to do that without making
> >>> schedutil provide the same level of performance in all of the relevant use
> >>> cases.
> >>
> >> Agreed; I though to have understood we were there already.
> >
> > AFAIK, not quite (added Giovanni as he has been paying more attention).
> > Schedutil has improved since it was merged but not to the extent where
> > it is a drop-in replacement. The standard it needs to meet is that
> > it is at least equivalent to powersave (in intel_pstate language)
> > or ondemand (acpi_cpufreq) and within a reasonable percentage of the
> > performance governor. Defaulting to performance is a) giving up and b)
> > the performance governor is not a universal win. There are some questions
> > currently on whether schedutil is good enough when HWP is not available.
> > 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 is a
> > hazard in itself because someone could test on the latest gen Intel CPU
> > and conclude everything is fine and miss that Intel-specific technology
> > is needed to make it work well while throwing everyone else under a bus.
> > Giovanni knows a lot more than I do about this, I could be wrong or
> > forgetting things.
> >
> > 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
> > 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.
>
> ..and it would be really useful for distros to know when the hard data
> is available so that they can make an informed decision when to move to
> schedutil.
>

I think distros are on the hook to generate that hard data themselves
with which to make such a decision. I don't expect it to be done by
someone else.

> >
> > For reference, SLUB had the same problem for years. It was switched
> > on by default in the kernel config but it was a long time before
> > SLUB was generally equivalent to SLAB in terms of performance. Block
> > multiqueue also had vaguely similar issues before the default changes
> > and a period of time before it was removed removed (example whinging mail
> > https://lore.kernel.org/lkml/20170803085115.r2jfz2lofy5spfdb@techsingularity.net/)
> > It's schedutil's turn :P
> >
>

Agreed. I'd like the option to switch back if we make the default change.
It's on the table and I'd like to be able to go that way.

Cheers,
Phil

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