Hi All,
Thanks a lot for the feedback so far!
One more respin after the last batch of comments from Peter and Frederic.
The previous summary that 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 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 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.
Patch 7 Deals with the situation in which the tick was stopped previously,
but the idle governor still predicts short idle.
This series is complementary to the poll_idle() patch at
https://patchwork.kernel.org/patch/10282237/
Thanks,
Rafael
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