On Tue, Jan 26, 2021 at 10:09:27AM +0100, Giovanni Gherdovich wrote: > On Mon, 2021-01-25 at 11:06 +0100, Peter Zijlstra wrote: > > On Fri, Jan 22, 2021 at 09:40:38PM +0100, Giovanni Gherdovich wrote: > > > 1. PROBLEM DESCRIPTION (over-utilization and schedutil) > > > > > > The problem happens on CPU-bound workloads spanning a large number of cores. > > > In this case schedutil won't select the maximum P-State. Actually, it's > > > likely that it will select the minimum one. > > > > > > A CPU-bound workload puts the machine in a state generally called > > > "over-utilization": an increase in CPU speed doesn't result in an increase of > > > capacity. The fraction of time tasks spend on CPU becomes constant regardless > > > of clock frequency (the tasks eat whatever we throw at them), and the PELT > > > invariant util goes up and down with the frequency (i.e. it's not invariant > > > anymore). > > > v5.10 v5.11-rc4 > > > ~~~~~~~~~~~~~~~~~~~~~~~~ > > > CPU activity (mpstat) 80-90% 80-90% > > > schedutil requests (tracepoint) always P0 mostly P2 > > > CPU frequency (HW feedback) ~2.2 GHz ~1.5 GHz > > > PELT root rq util (tracepoint) ~825 ~450 > > > > > > mpstat shows that the workload is CPU-bound and usage doesn't change with > > > > So I'm having trouble with calling a 80%-90% workload CPU bound, because > > clearly there's a ton of idle time. > > Yes you're right. There is considerable idle time and calling it CPU-bound is > a bit of a stretch. > > Yet I don't think I'm completely off the mark. The busy time is the same with > the machine running at 1.5 GHz and at 2.2 GHz (it just takes longer to > finish). To me it seems like the CPU is the bottleneck, with some overhead on > top. > I think this is an important observation because while the load may not be fully CPU-bound, it's still at the point where race-to-idle by running at a higher frequency is relevant. During the busy time, the results (and Michael's testing) indicate that the higher frequency may still be justified. I agree that there is a "a 'problem' between schedutil and cpufreq, they don't use the same f_max at all times", fixing that mid -rc may not be appropriate because it's a big change in an rc window. So, should this patch be merged for 5.11 as a stopgap, fix up schedutil/cpufreq and then test both AMD and Intel chips reporting the correct max non-turbo and max-turbo frequencies? That would give time to give some testing in linux-next before merging to reduce the chance something else falls out. -- Mel Gorman SUSE Labs
