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@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. Cheers, Phil --
