[PATCH v2 0/1] Forced-wakeup for stop states on Powernv

[Abhishek Goel <huntbag@linux.vnet.ibm.com>]

Currently, the cpuidle governors determine what idle state a idling CPU
should enter into based on heuristics that depend on the idle history on
that CPU. Given that no predictive heuristic is perfect, there are cases
where the governor predicts a shallow idle state, hoping that the CPU will
be busy soon. However, if no new workload is scheduled on that CPU in the
near future, the CPU will end up in the shallow state.

Motivation
----------
In case of POWER, this is problematic, when the predicted state in the
aforementioned scenario is a shallow stop state on a tickless system. As
we might get stuck into shallow states even for hours, in absence of ticks
or interrupts.

To address this, We forcefully wakeup the cpu by setting the decrementer.
The decrementer is set to a value that corresponds with the residency of
the next available state. Thus firing up a timer that will forcefully
wakeup the cpu. Few such iterations will essentially train the governor to
select a deeper state for that cpu, as the timer here corresponds to the
next available cpuidle state residency. Thus, cpu will eventually end up
in the deepest possible state and we won't get stuck in a shallow state
for long duration.

Experiment
----------
For earlier versions when this feature was meat to be only for shallow lite
states, I performed experiments for three scenarios to collect some data.

case 1 :
Without this patch and without tick retained, i.e. in a upstream kernel,
It would spend more than even a second to get out of stop0_lite.

case 2 : With tick retained in a upstream kernel -

Generally, we have a sched tick at 4ms(CONF_HZ = 250). Ideally I expected
it to take 8 sched tick to get out of stop0_lite. Experimentally,
observation was

=========================================================
sample          min            max           99percentile
20              4ms            12ms          4ms
=========================================================

It would take atleast one sched tick to get out of stop0_lite.

case 2 :  With this patch (not stopping tick, but explicitly queuing a
          timer)

============================================================
sample          min             max             99percentile
============================================================
20              144us           192us           144us
============================================================


Description of current implementation
-------------------------------------

We calculate timeout for the current idle state as the residency value
of the next available idle state. If the decrementer is set to be
greater than this timeout, we update the decrementer value with the
residency of next available idle state. Thus, essentially training the
governor to select the next available deeper state until we reach the
deepest state. Hence, we won't get stuck unnecessarily in shallow states
for longer duration.

--------------------------------
v1 of auto-promotion : https://lkml.org/lkml/2019/3/22/58 This patch was
implemented only for shallow lite state in generic cpuidle driver.

v2 of auto-promotion : Removed timeout_needed and rebased to current
upstream kernel

Then, 
v1 of forced-wakeup : Moved the code to cpuidle powernv driver and started
as forced wakeup instead of auto-promotion

v2 of forced-wakeup : Extended the forced wakeup logic for all states.
Setting the decrementer instead of queuing up a hrtimer to implement the
logic.

Abhishek Goel (1):
  cpuidle-powernv : forced wakeup for stop states

 drivers/cpuidle/cpuidle-powernv.c | 38 +++++++++++++++++++++++++++++++
 1 file changed, 38 insertions(+)

-- 
2.17.1