Messages in this thread | | | From | "Rafael J. Wysocki" <> | Date | Tue, 18 Jun 2019 10:36:56 +0200 | Subject | Re: [PATCH] sched/fair: Introduce fits_capacity() |
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On Tue, Jun 18, 2019 at 10:25 AM Quentin Perret <quentin.perret@arm.com> wrote: > > On Tuesday 18 Jun 2019 at 10:10:48 (+0200), Rafael J. Wysocki wrote: > > On Tue, Jun 18, 2019 at 9:47 AM Viresh Kumar <viresh.kumar@linaro.org> wrote: > > > > > > On 18-06-19, 09:26, Rafael J. Wysocki wrote: > > > > On Tue, Jun 18, 2019 at 5:12 AM Viresh Kumar <viresh.kumar@linaro.org> wrote: > > > > > > > > > > +Rafael > > > > > > > > > > On 17-06-19, 17:02, Peter Zijlstra wrote: > > > > > > On Thu, Jun 06, 2019 at 08:22:04AM +0530, Viresh Kumar wrote: > > > > > > > Hmm, even if the values are same currently I am not sure if we want > > > > > > > the same for ever. I will write a patch for it though, if Peter/Rafael > > > > > > > feel the same as you. > > > > > > > > > > > > Is it really the same variable or just two numbers that happen to be the > > > > > > same? > > > > > > > > > > In both cases we are trying to keep the load under 80% of what can be supported. > > > > > But I am not sure of the answer to your question. > > > > > > > > > > Maybe Rafael knows :) > > > > > > > > Which variable? > > > > > > Schedutil multiplies the target frequency by 1.25 (20% more capacity eventually) > > > to get enough room for more load and similar thing is done in fair.c at several > > > places to see if the new task can fit in a runqueue without overloading it. > > > > For the schedutil part, see the changelog of the commit that introduced it: > > > > 9bdcb44e391d cpufreq: schedutil: New governor based on scheduler > > utilization data > > > > As for the other places, I don't know about the exact reasoning. > > > > > Quentin suggested to use common code for this calculation and that is what is > > > getting discussed here. > > > > I guess if the rationale for the formula is the same in all cases, it > > would be good to consolidate that code and document the rationale > > while at it. > > I _think_ it is, but I guess others could correct me if this is > incorrect. > > When choosing a CPU or a frequency using a util value, we look for a > capacity that will provide us with 20% of idle time. And in both case we > use the same threshold, just hardcoded in different places. Hence the > suggestion to unify things. > > I hope that makes sense :-)
Well, for schedutil, the 1.25 value comes from the case when utilization is not frequency-invariant the next-frequency formula is recursive (the next frequency is proportional to the current one). It is chosen to get the new frequency equal to the old one if (util / max) is .8. That translates to the "capacity that will provide 20% more of idle time" in the frequency-invariant utilization case, but the original rationale was different.
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