[RFC/RFT][PATCH v3 0/6] sched/cpuidle: Idle loop rework

[Rafael J. Wysocki]

Hi All,

Thanks a lot for the discussion and testing so far!

This is a total respin of the whole series, so please look at it afresh.
Patches 2 and 3 are the most similar to their previous versions, but
still they are different enough.

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 subsequent changes.

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.  It also adds code to decide what value to
return as "nohz" to the menu governor.

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 causes the menu governor to refine the state selection in case the
tick is not going to be stopped and the already selected state may not fit
before the next tick time.

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 above said, this is just RFC, so no pets close to the machines running it,
> please.

The patches are on top of 4.16-rc4 (if you need a git branch with them for
easier testing, please let me know).

Thanks,
Rafael