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). Other distros assuming they're watching can nominate their own victim. -- Mel Gorman SUSE Labs
