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SubjectRe: [PATCH v2] sched/clock: print a log when the sched clock is marked unstable


On Wed, 7 Jul 2021, Peter Zijlstra wrote:

> On Wed, Jul 07, 2021 at 04:26:59PM +0530, Ani Sinha wrote:
> > When the sched clock transitions from stable to unstable and
> > vice versa, a kernel log is printed. When the sched clock
> > is marked explicitly as unstable, make __clear_sched_clock_stable()
> > emit a warning log. It is useful for example in understanding
> > why a certain feature like NOHZ that depends on availability of a
> > stable sched clock, is not available.
> >
> > Signed-off-by: Ani Sinha <ani@anisinha.ca>
> > ---
> > kernel/sched/clock.c | 1 +
> > 1 file changed, 1 insertion(+)
> >
> > Changelog:
> > V1: original patch
> > v2: print pr_warn from __clear_sched_clock_stable instead. Commit log
> > updated with proper prefix and wording.
> >
> > diff --git a/kernel/sched/clock.c b/kernel/sched/clock.c
> > index c2b2859ddd82..99484fec0335 100644
> > --- a/kernel/sched/clock.c
> > +++ b/kernel/sched/clock.c
> > @@ -183,6 +183,7 @@ static void __clear_sched_clock_stable(void)
> > return;
> >
> > tick_dep_set(TICK_DEP_BIT_CLOCK_UNSTABLE);
> > + pr_warn("sched_clock: Marking unstable.\n");
> > schedule_work(&sched_clock_work);
> > }
>
> I'm confused... that work has a ton of printk()s in, what is this
> pr_warn() adding?
>

Right, but for whatever reason, on latest version of Ubuntu (21.04)
running on hyperV, I do not see those logs:

azureuser@CentOS:~$ uname -a
Linux CentOS 5.11.0-1009-azure #9-Ubuntu SMP Thu Jun 17 02:20:07 UTC 2021
x86_64 x86_64 x86_64 GNU/Linux
azureuser@CentOS:~$ dmesg | grep unstable
dmesg: read kernel buffer failed: Operation not permitted
azureuser@CentOS:~$ sudo su
root@CentOS:/home/azureuser# dmesg | grep unstable
[ 0.000000] tsc: Marking TSC unstable due to running on Hyper-V
root@CentOS:/home/azureuser# dmesg | grep sched
[ 0.384656] rcu: RCU calculated value of scheduler-enlistment delay is
25 jiffies.
[ 1.122977] io scheduler mq-deadline registered
root@CentOS:/home/azureuser# exit

Hence, I was trying the patch.

A

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