Messages in this thread | | | From | "Rafael J. Wysocki" <> | Subject | Re: [RFC][PATCH 2/2] cpufreq: schedutil: Force max frequency on busy CPUs | Date | Mon, 20 Mar 2017 13:35:12 +0100 |
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On Monday, March 20, 2017 11:36:45 AM Peter Zijlstra wrote: > On Sun, Mar 19, 2017 at 02:34:32PM +0100, Rafael J. Wysocki wrote: > > From: Rafael J. Wysocki <rafael.j.wysocki@intel.com> > > > > The PELT metric used by the schedutil governor underestimates the > > CPU utilization in some cases. The reason for that may be time spent > > in interrupt handlers and similar which is not accounted for by PELT. > > > > That can be easily demonstrated by running kernel compilation on > > a Sandy Bridge Intel processor, running turbostat in parallel with > > it and looking at the values written to the MSR_IA32_PERF_CTL > > register. Namely, the expected result would be that when all CPUs > > were 100% busy, all of them would be requested to run in the maximum > > P-state, but observation shows that this clearly isn't the case. > > The CPUs run in the maximum P-state for a while and then are > > requested to run slower and go back to the maximum P-state after > > a while again. That causes the actual frequency of the processor to > > visibly oscillate below the sustainable maximum in a jittery fashion > > which clearly is not desirable. > > > > To work around this issue use the observation that, from the > > schedutil governor's perspective, CPUs that are never idle should > > always run at the maximum frequency and make that happen. > > > > To that end, add a counter of idle calls to struct sugov_cpu and > > modify cpuidle_idle_call() to increment that counter every time it > > is about to put the given CPU into an idle state. Next, make the > > schedutil governor look at that counter for the current CPU every > > time before it is about to start heavy computations. If the counter > > has not changed for over SUGOV_BUSY_THRESHOLD time (equal to 50 ms), > > the CPU has not been idle for at least that long and the governor > > will choose the maximum frequency for it without looking at the PELT > > metric at all. > > Why the time limit?
One iteration appeared to be a bit too aggressive, but honestly I think I need to check again if this thing is regarded as viable at all.
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