Messages in this thread | | | From | Ani Sinha <> | Date | Sat, 10 Jul 2021 09:19:47 +0530 | Subject | Re: [PATCH v2] sched/clock: print a log when the sched clock is marked unstable |
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On Wed, Jul 7, 2021 at 5:48 PM Ani Sinha <ani@anisinha.ca> 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@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. > > 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.
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