On 21/10/2020 09:26, Peter Zijlstra wrote: > On Tue, Oct 20, 2020 at 05:44:38PM +0200, Redha Gouicem wrote: >> The first patch of the series is not specific to scheduling. It allows us >> (or anyone else) to use the cpufreq infrastructure at a different sampling >> rate without compromising the cpufreq subsystem and applications that >> depend on it. > It's also completely redudant as the scheduler already reads aperf/mperf > on every tick. Clearly you didn't do your homework ;-) My bad. I worked on this a year ago, just never got time to submit to the lkml and I should have re-done my homework more thoroughly before submitting. The paper was submitted approximately at the same time as the patch introducing support frequency invariance and frequency reading at every tick (1 week apart!) Again, my bad. > >> The main idea behind this patch series is to bring to light the frequency >> inversion problem that will become more and more prominent with new CPUs >> that feature per-core DVFS. The solution proposed is a first idea for >> solving this problem that still needs to be tested across more CPUs and >> with more applications. > Which is why schedutil (the only cpufreq gov anybody should be using) is > integrated with the scheduler and closes the loop and tells the CPU > about the expected load. > While I agree that schedutil is probably a good option, I'm not sure we treat exactly the same problem. schedutil aims at mapping the frequency of the CPU to the actual load. What I'm saying is that since it takes some time for the frequency to match the load, why not account for the frequency when making placement/migration decisions. I know that with the frequency invariance code, capacity accounts for frequency, which means that thread placement decisions do account for frequency indirectly. However, we still have performance improvements with our patch for the workloads with fork/wait patterns. I really believe that we can still gain performance if we make decisions while accounting for the frequency more directly.
