[RFC PATCH v2 0/8] Introduct cpu idle prediction functionality

Aubrey Li
2017-09-30T07:21:32Z

We found under some latency intensive workloads, short idle periods occurs
very common, then idle entry and exit path starts to dominate, so it's
important to optimize them. To determine the short idle pattern, we need
to figure out how long of the coming idle and the threshold of the short
idle interval.

A cpu idle prediction functionality is introduced in this proposal to catch
the short idle pattern.

Firstly, we check the IRQ timings subsystem, if there is an event
coming soon.
-- https://lwn.net/Articles/691297/

Secondly, we check the idle statistics of scheduler, if it's likely we'll
go into a short idle.
-- https://patchwork.kernel.org/patch/2839221/

Thirdly, we predict the next idle interval by using the prediction
fucntionality in the idle governor if it has.

For the threshold of the short idle interval, we record the timestamps of
the idle entry, and multiply by a tunable parameter at here:
-- /proc/sys/kernel/fast_idle_ratio

We use the output of the idle prediction to skip turning tick off if a
short idle is determined in this proposal. Reprogramming hardware timer
twice(off and on) is expensive for a very short idle. There are some
potential optimizations can be done according to the same indicator.

I observed when system is idle, the idle predictor reports 20/s long idle
and ZERO fast idle on one CPU. And when the workload is running, the idle
predictor reports 72899/s fast idle and ZERO long idle on the same CPU.

Aubrey Li (8):
cpuidle: menu: extract prediction functionality
cpuidle: record the overhead of idle entry
cpuidle: add a new predict interface
tick/nohz: keep tick on for a fast idle
timers: keep sleep length updated as needed
cpuidle: make fast idle threshold tunable
cpuidle: introduce irq timing to make idle prediction
cpuidle: introduce run queue average idle to make idle prediction

drivers/cpuidle/Kconfig | 1 +
drivers/cpuidle/cpuidle.c | 109 +++++++++++++++++++++++++++++++++++++++
drivers/cpuidle/governors/menu.c | 69 ++++++++++++++++---------
include/linux/cpuidle.h | 21 ++++++++
kernel/sched/idle.c | 14 ++++-
kernel/sysctl.c | 12 +++++
kernel/time/tick-sched.c | 7 +++
7 files changed, 209 insertions(+), 24 deletions(-)

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2.7.4