linux-kernel - Re: [RFC PATCH v2 0/8] Introduct cpu idle prediction functionality

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Message-ID: <3026355.QRuoy6eIZM@aspire.rjw.lan>
Date:   Sat, 14 Oct 2017 03:14:10 +0200
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Aubrey Li <aubrey.li@...el.com>
Cc:     tglx@...utronix.de, peterz@...radead.org, len.brown@...el.com,
        ak@...ux.intel.com, tim.c.chen@...ux.intel.com, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v2 0/8] Introduct cpu idle prediction functionality

On Saturday, September 30, 2017 9:20:26 AM CEST Aubrey Li wrote:
> 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(-)
> 

Overall, it looks like you could avoid stopping the tick every time the
predicted idle duration is not longer than the tick interval in the first
place.

Why don't you do that?