Re: [PATCH v11] cpuidle: New timer events oriented governor for tickless systems

["Rafael J. Wysocki" <rafael@kernel.org>]

On Wed, Jan 16, 2019 at 1:22 PM Daniel Lezcano
<daniel.lezcano@linaro.org> wrote:
>
> On 16/01/2019 13:03, Rafael J. Wysocki wrote:
> > On Friday, January 4, 2019 12:30:47 PM CET Rafael J. Wysocki wrote:
> >> From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> >>
> >> The venerable menu governor does some things that are quite
> >> questionable in my view.
> >>
> >> First, it includes timer wakeups in the pattern detection data and
> >> mixes them up with wakeups from other sources which in some cases
> >> causes it to expect what essentially would be a timer wakeup in a
> >> time frame in which no timer wakeups are possible (because it knows
> >> the time until the next timer event and that is later than the
> >> expected wakeup time).
> >>
> >> Second, it uses the extra exit latency limit based on the predicted
> >> idle duration and depending on the number of tasks waiting on I/O,
> >> even though those tasks may run on a different CPU when they are
> >> woken up.  Moreover, the time ranges used by it for the sleep length
> >> correction factors depend on whether or not there are tasks waiting
> >> on I/O, which again doesn't imply anything in particular, and they
> >> are not correlated to the list of available idle states in any way
> >> whatever.
> >>
> >> Also, the pattern detection code in menu may end up considering
> >> values that are too large to matter at all, in which cases running
> >> it is a waste of time.
> >>
> >> A major rework of the menu governor would be required to address
> >> these issues and the performance of at least some workloads (tuned
> >> specifically to the current behavior of the menu governor) is likely
> >> to suffer from that.  It is thus better to introduce an entirely new
> >> governor without them and let everybody use the governor that works
> >> better with their actual workloads.
> >>
> >> The new governor introduced here, the timer events oriented (TEO)
> >> governor, uses the same basic strategy as menu: it always tries to
> >> find the deepest idle state that can be used in the given conditions.
> >> However, it applies a different approach to that problem.
> >>
> >> First, it doesn't use "correction factors" for the time till the
> >> closest timer, but instead it tries to correlate the measured idle
> >> duration values with the available idle states and use that
> >> information to pick up the idle state that is most likely to "match"
> >> the upcoming CPU idle interval.
> >>
> >> Second, it doesn't take the number of "I/O waiters" into account at
> >> all and the pattern detection code in it avoids taking timer wakeups
> >> into account.  It also only uses idle duration values less than the
> >> current time till the closest timer (with the tick excluded) for that
> >> purpose.
> >
> > Given the lack of negative feedback and my confidence in this, I'm
> > queuing it up for 5.1.
>
> Yeah, this governor was not proposed in the better moment for review for
> me :/
>
> I agree writing a new governor is certainly simpler than revisiting the
> menu governor again. That is the advantage of the governors, we can add
> more of them suiting the needs.
>
> If you are confident and Doug bench-marked it showing better results
> than the menu governor on x86, I don't see any reason to not merge it.
>
> Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>

Thank you!