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

[Ani Sinha]

On Wed, Jul 7, 2021 at 5:48 PM Ani Sinha <ani@xxxxxxxxxxx> wrote:
>
>
>
> On Wed, 7 Jul 2021, Ani Sinha wrote:
>
> >
> >
> > 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@xxxxxxxxxxx>
> > > > ---
> > > >  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.
>
> OK never mimd. Seems on hyperV, they use read_hv_sched_clock_tsc() for
> pv_sched_clock and therefore using_native_sched_clock() returns false.
> Thus, clear_sched_clock_stable() is not called.
>
> I will need to do some instrumentation to confirm this. Maybe we do not
> need any patching after all.

Yes, no additional logging is needed here. This patch can be
discarded. There is a potential issue on the platform side code for
hyperV for which we might send a separate patch.