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SubjectRe: [RFC][PATCH 2/2] cpufreq: schedutil: Force max frequency on busy CPUs
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On Monday, March 20, 2017 01:06:15 PM Patrick Bellasi wrote:
> On 20-Mar 13:50, Peter Zijlstra wrote:
> > On Mon, Mar 20, 2017 at 01:35:12PM +0100, Rafael J. Wysocki wrote:
> > > 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.
> > >
> >
> > I don't hate the idea; if we don't hit idle; we shouldn't shift down. I
> > just wonder if we don't already keep a idle-seqcount somewhere; NOHZ and
> > RCU come to mind as things that might already use something like that.
>
> Maybe the problem is not going down (e.g. when there are only small
> CFS tasks it makes perfectly sense) but instead not being fast enough
> on rampin-up when a new RT task is activated.
>
> And this boils down to two main point:
> 1) throttling for up transitions perhaps is only harmful
> 2) the call sites for schedutils updates are not properly positioned
> in specific scheduler decision points.
>
> The proposed patch is adding yet another throttling mechanism, perhaps
> on top of one which already needs to be improved.

It is not throttling anything.

Thanks,
Rafael

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