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

[Mel Gorman <mgorman@suse.de>]

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@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. 
> 

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

-- 
Mel Gorman
SUSE Labs