Re: [RFC][PATCH 2/2] cpufreq: schedutil: Force max frequency on busy CPUs

[Peter Zijlstra]

On Sun, Mar 19, 2017 at 02:34:32PM +0100, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> 
> 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?