Re: [RFC/RFT][PATCH v6] cpuidle: New timer events oriented governor for tickless systems

Rafael J. Wysocki
2018-11-23T10:40:51Z

On Friday, November 23, 2018 11:35:38 AM CET Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
>
> The venerable menu governor does some thigns 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 (becuase 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.
>
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> ---
>
> v5 -> v6:
> * Avoid applying poll_time_limit to non-polling idle states by mistake.
> * Use idle duration measured by the governor for everything (as it likely is
> more accurate than the one measured by the core).

This particular change is actually missing, sorry about that. It is not
essential, however, so the v6 should be good enough as is for evaluation
and review purposes.

> * Rename SPIKE to PULSE.
> * Do not run pattern detection upfront. Instead, use recent idle duration
> values to refine the state selection after finding a candidate idle state.
> * Do not use the expected idle duration as an extra latency constraint
> (exit latency is less than the target residency for all of the idle states
> known to me anyway, so this doesn't change anything in practice).

Thanks,
Rafael