[RFC/RFT][PATCH v2 0/6] sched/cpuidle: Idle loop rework

[Rafael J. Wysocki]

Hi All,

Thanks a lot for the discussion so far!

Here's a new version of the series addressing some comments from the
discussion and (most importantly) replacing patches 4 and 5 with another
(simpler) patch.

The summary below still applies:

On Sunday, March 4, 2018 11:21:30 PM CET Rafael J. Wysocki wrote:
> 
> The problem is that if we stop the sched tick in
> tick_nohz_idle_enter() and then the idle governor predicts short idle
> duration, we lose regardless of whether or not it is right.
> 
> If it is right, we've lost already, because we stopped the tick
> unnecessarily.  If it is not right, we'll lose going forward, because
> the idle state selected by the governor is going to be too shallow and
> we'll draw too much power (that has been reported recently to actually
> happen often enough for people to care).
> 
> This patch series is an attempt to improve the situation and the idea
> here is to make the decision whether or not to stop the tick deeper in
> the idle loop and in particular after running the idle state selection
> in the path where the idle governor is invoked.  This way the problem
> can be avoided, because the idle duration predicted by the idle governor
> can be used to decide whether or not to stop the tick so that the tick
> is only stopped if that value is large enough (and, consequently, the
> idle state selected by the governor is deep enough).
> 
> The series tires to avoid adding too much new code, rather reorder the
> existing code and make it more fine-grained.
> 
> 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 it 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 changes in patch 6.

Patch 4 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.

Patch 5 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.

Patch 6 cleans up the code to avoid running one piece of it twice in a row
in some cases.

And the two paragraphs below still apply:

> I have tested these patches on a couple of machines, including the very laptop
> I'm sending them from, without any obvious issues, but please give them a go
> if you can, especially if you have an easy way to reproduce the problem they
> are targeting.  The patches are on top of 4.16-rc3 (if you need a git branch
> with them for easier testing, please let me know).
> 
> The above said, this is just RFC, so no pets close to the machines running it,
> please, and I'm kind of expecting Peter and Thomas to tear it into pieces. :-)

Thanks,
Rafael