linux-kernel - Re: [PATCH v1] Print a log when the sched clock is marked unstable

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Date:   Wed, 7 Jul 2021 16:35:07 +0530 (IST)
From:   Ani Sinha <ani@...sinha.ca>
To:     Thomas Gleixner <tglx@...utronix.de>
cc:     Ani Sinha <ani@...sinha.ca>, linux-kernel@...r.kernel.org,
        anirban.sinha@...ia.com, Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>
Subject: Re: [PATCH v1] Print a log when the sched clock is marked unstable


Tglx,

On Wed, 7 Jul 2021, Thomas Gleixner wrote:

> Ani,
>
> On Sun, Jun 27 2021 at 12:19, Ani Sinha wrote:
>
> I asked you to read Documentation/process carefully and I told you:
>
>         Also your subject line want's a proper prefix.
>
> Your patch is missing a prefix again. Hint:
>
>   git log kernel/sched/clock.c
>

OK thanks for the pointer. Its my bad that staring at the computer for so
many hours stuck at home makes me read "prefix" as "fix". So I read

Also your subject line want's a proper fix


> > In other parts of the kernel when the sched clock transitions from
> > stable to unstable and vice versa, a kernel info log is printed.  When
> > the sched clock is marked explicitly as unstable, we should print an
> > information log as well. This patch addresses this. It is useful in
>
> git grep 'This patch' Documentation/process/
>
> Also please avoid the 'we' wording. 'We should print' does not make
> sense if you really think about it.

Please understand that Linux kernel is not the only upstream software
where I am contrinuting. Each community has their own rules. Maybe add a
check in checkpatch.pl in the kernel so that these things maybe caught
automatically?

>
> > cases where for example, we want to understand why a certain feature
> > like NOHZ, which depends on a stable sched clock, is not available.
> >
> > Signed-off-by: Ani Sinha <ani@...sinha.ca>
> > ---
> >  kernel/sched/clock.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > See also thread
> > https://lkml.org/lkml/2021/6/20/32
> >
> > diff --git a/kernel/sched/clock.c b/kernel/sched/clock.c
> > index c2b2859ddd82..5a403b2c9b3f 100644
> > --- a/kernel/sched/clock.c
> > +++ b/kernel/sched/clock.c
> > @@ -192,8 +192,10 @@ void clear_sched_clock_stable(void)
> >
> >  	smp_mb(); /* matches sched_clock_init_late() */
> >
> > -	if (static_key_count(&sched_clock_running.key) == 2)
> > +	if (static_key_count(&sched_clock_running.key) == 2) {
> > +		pr_info("sched_clock: Marking unstable.\n");
> >  		__clear_sched_clock_stable();
>
> Why isn't that in __clear_sched_clock_stable() ?
>

updated in v2. Makes sense do emit the log at the lowest level of
abstraction so that all code paths are potentially caught.

Ani