linux-kernel - Re: [PATCH] cpufreq: stats: Switch to ktime and msec instead of jiffies and usertime

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Date:   Wed, 11 Nov 2020 15:21:07 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Thomas Renninger <trenn@...e.de>
Cc:     Rafael Wysocki <rjw@...ysocki.net>,
        Jonathan Corbet <corbet@....net>, linux-pm@...r.kernel.org,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Shuah Khan <shuah@...nel.org>, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] cpufreq: stats: Switch to ktime and msec instead of
 jiffies and usertime

On 11-11-20, 09:13, Thomas Renninger wrote:
> Am Mittwoch, 11. November 2020, 06:13:50 CET schrieb Viresh Kumar:
> > On 10-11-20, 13:53, Thomas Renninger wrote:
> > > Am Dienstag, 10. November 2020, 12:07:37 CET schrieb Viresh Kumar:
> > > > The cpufreq and thermal core, both provide sysfs statistics to help
> > > > userspace learn about the behavior of frequencies and cooling states.
> > > > 
> > > > This is how they look:
> > > > /sys/devices/system/cpu/cpufreq/policy0/stats/time_in_state:1200000 399
> > > > 
> > > > The results look like this after this commit:
> > > > /sys/devices/system/cpu/cpufreq/policy0/stats/time_in_state:1200000 3830
> > > 
> > > How would userspace know whether it's ms or 10ms?
> 
> Again:
> How would userspace know whether it's ms or 10ms?

Yeah, I understand the problem you are pointing at.

> > > whatabout a new file with the same convention as cooling devices (adding ms):
> > Keeping two files for same stuff is not great, and renaming the file
> > breaks userspace ABI.
> 
> No exactly the other way around:
> - Renaming, breaks the userspace ABI.
> - Two files would be the super correct way to go:

Yes, but then this is just some stats which a very limited number of
people should be using and so ...

>   - Deprecate the old file and keep the 10ms around for some years
>     ./Documentation/ABI/obsolete
>   - Add the new interface and document it in:
>    ./Documentation/ABI/testing
> 
> As this is about a minor cpufreq_stat debug file, it is enough if
> you rename to:
> > /sys/devices/system/cpu/cpufreq/policy0/stats/time_in_state_ms

... I agree about this. Just rename the file accordingly. Which will
also make sure that everyone follows that something got changed in the
kernel.

> > I already fixed this recently and stats don't appear empty for fast
> > switch anymore.
> 
> Then cpufreq_stats could be a module again?

No, not really. This is some code that needs to get called from
cpufreq core, without any notifiers and as fast as possible as we may
be in scheduler's hot path. So the module thing isn't going to work
now.

-- 
viresh