linux-kernel - Re: [PATCH v2] cpuidle: Use nanoseconds as the unit of time

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Date:   Fri, 8 Nov 2019 10:45:03 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Doug Smythies <dsmythies@...us.net>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Giovanni Gherdovich <ggherdovich@...e.cz>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Linux PM <linux-pm@...r.kernel.org>
Subject: Re: [PATCH v2] cpuidle: Use nanoseconds as the unit of time

On Fri, Nov 8, 2019 at 9:45 AM Doug Smythies <dsmythies@...us.net> wrote:
>
> On 2019.11.07 17:44 Rafael J. Wysocki wrote:
> > On Thu, Nov 7, 2019 at 3:25 PM Rafael J. Wysocki <rjw@...ysocki.net> wrote:
> >>
> >> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> >>
> >> Currently, the cpuidle subsystem uses microseconds as the unit of
> >> time which (among other things) causes the idle loop to incur some
> >> integer division overhead for no clear benefit.
> >>
> >> In order to allow cpuidle to measure time in nanoseconds, add two
> >> additional fields, exit_latency_ns and target_residency_ns, to
> >> represent the exit latency and target residency of an idle state
> >> in nanoseconds, respectively, to struct cpuidle_state_usage and
> >> initialize them with the help of the corresponding values in
> >> microseconds provided by drivers.  In addition to that, change
> >> cpuidle_governor_latency_req() to return the idle state exit
> >> latency constraint in nanoseconds.
> >>
> >> With that, meeasure idle state residency (last_residency_ns in
> >> struct cpuidle_device and time_ns in struct cpuidle_driver) in
> >> nanoseconds and update the cpuidle core and governors accordingly.
> >>
> >> However, the menu governor still computes typical intervals in
> >> microseconds to avoid integer overflows.
> >
> > Since this addresses all of the comments received by the RFC version
> > that was posted over a month ago and I don't see any more issues with
> > it, I'm tempted to simply queue it up for 5.5 unless somebody sees a
> > good enough reason why that would be a bad idea.
>
> Could I please have another day or two?

Sure, it won't go straight into linux-next anyway. :-)

> I did try the RFC version, but not much as I went off on those
> teo issues and backtracked pretty quickly.
>
> I have been running this v2 today, with both menu and teo
> governors. Acquiring some baseline reference data to compare
> to now. The menu governor response seems different (Supporting
> information/graphs will come later).

That may be good or bad, depending in what way it is different. :-)

> teo just started.
>
> I lost a bunch of time due to being somewhat linux-next challenged.

No worries, please take your time!

I very much appreciate the testing work you are doing.

Cheers!