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

[Rafael J. Wysocki]

On Thu, Oct 22, 2020 at 6:35 PM Mel Gorman <mgorman@xxxxxxx> wrote:
>
> On Thu, Oct 22, 2020 at 11:12:00AM -0400, Phil Auld wrote:
> > > > 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.
> >
>
> Yep, distros are on the hook. When I said "I would expect hard data",
> it was in the knowledge that for openSUSE/SLE, we (as in SUSE) would be
> generating said data and making a call based on it. I'd be surprised if
> Phil was not thinking along the same lines.
>
> > > > 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@xxxxxxxxxxxxxxxxxxx/)
> > > > 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.
> >
>
> Yep. It sounds chicken, but it's a useful safety net and a reasonable
> way to deprecate a feature. It's also useful for bug creation -- User X
> running whatever found that schedutil is worse than the old governor and
> had to temporarily switch back. Repeat until complaining stops and then
> tear out the old stuff.
>
> When/if there is a patch setting schedutil as the default, cc suitable
> distro people (Giovanni and myself for openSUSE).

So for the record, Giovanni was on the CC list of the "cpufreq:
intel_pstate: Use passive mode by default without HWP" patch that this
discussion resulted from (and which kind of belongs to the above
category).

> Other distros assuming they're watching can nominate their own victim.

But no other victims had been nominated at that time.