[PATCH v9 00/10] sched/cpuidle: Idle loop rework

Rafael J. Wysocki
2018-04-04T08:53:25Z

Hi All,

Thanks a lot for the feedback so far!

For the motivation/summary, please refer to the BZ entry at

https://bugzilla.kernel.org/show_bug.cgi?id=199227

created for collecting information related to this patch series. Some v7.3
testing results from Len and Doug are in there already.

The testing so far shows significant idle power improvements, both in terms of
reducing the average idle power (about 10% on some systems) and in terms of
reducing the idle power noise (in the vast majority of cases, with this series
applied the idle power is mostly stable around the power floor of the system).
The average power is also reduced in some non-idle workloads and there are
some performance improvements in them.

It also is reported that the series generally addresses the problem it has been
motivated by (ie. the "powernightmares" issue).

This revision is mostly a re-send of the v8 with three patches changed as
follows.

> Patch 1 prepares the tick-sched code for the subsequent modifications and it
> doesn't change the code's functionality (at least not intentionally).
>
> Patch 2 starts pushing the tick stopping decision deeper into the idle
> loop, but that is limited to do_idle() and tick_nohz_irq_exit().
>
> Patch 3 makes cpuidle_idle_call() decide whether or not to stop the tick
> and sets the stage for the subsequent changes.
>
> Patch 4 is a new one just for the TICK_USEC definition changes.
>
> Patch 5 adds a bool pointer argument to cpuidle_select() and the ->select
> governor callback allowing them to return a "nohz" hint on whether or not to
> stop the tick to the caller. It also adds code to decide what value to
> return as "nohz" to the menu governor and modifies its correction factor
> computations to take running tick into account if need be.
>
> Patch 6 (which is new) contains some changes that previously were included
> into the big reordering patch (patch [6/8] in the v7). Essentially, it does
> more tick-sched code reorganization in preparation for the subsequent changes
> (and should not modify the functionality).

Patch 7 is a new version of its v8 counterpart. It makes fewer changes to the
existing code and adds a special function for the handling of the use case it
is about. It still makes some hrtimer code modifications allowing it to return
the time to the next event with one timer excluded (which needs to be done with
respect to the tick timer), though.

Patch 8 reorders the idle state selection with respect to the stopping of
the tick and causes the additional "nohz" hint from cpuidle_select() to be
used for deciding whether or not to stop the tick. It is a rebased version
of its v8 counterpart.

Patch 9 causes the menu governor to refine the state selection in case the
tick is not going to be stopped and the already selected state does not fit
the interval before the next tick time. It is a new version that avoids
using state 0 if it has been disabled (if state 0 has been disabled, the
governor only should use it when no states are enabled at all).

> Patch 10 Deals with the situation in which the tick was stopped previously,
> but the idle governor still predicts short idle (it has not changed).

This series is complementary to the poll_idle() patches discussed recently

https://patchwork.kernel.org/patch/10282237/
https://patchwork.kernel.org/patch/10311775/

that have been merged for v4.17 already.

There is a new git branch containing the current series at

git://git.kernel.org/pub/scm/linux/kernel/git/rafael/linux-pm.git \
idle-loop-v9

Thanks,
Rafael