Messages in this thread | | | Date | Mon, 20 Mar 2017 11:36:45 +0100 | From | Peter Zijlstra <> | Subject | Re: [RFC][PATCH 2/2] cpufreq: schedutil: Force max frequency on busy CPUs |
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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?
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