[GIT PULL clocksource] Clocksource watchdog commits for v5.14

[Paul E. McKenney]

Hello, Thomas,

This pull request contains changes to cause the clocksource watchdog to
reject measurement noise caused by delays between clocksource reads.
These have been posted a few times to LKML, most recently v15:

https://lore.kernel.org/lkml/20210527190042.GA438700@paulmck-ThinkPad-P17-Gen-1/

These have been acked by Feng Tang and reviewed by Luming Yu.  These have
also been subjected to subjected to the kbuild test robot and -next
testing, and are available in the git repository based on v5.13-rc1 at:

  git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git clocksource

for you to fetch changes up to 023766fbdde49c75c13acd268bf9af810d624c59:

  clocksource: Print deviation in nanoseconds for unstable case (2021-06-04 13:20:31 -0700)

----------------------------------------------------------------
Feng Tang (1):
      clocksource: Print deviation in nanoseconds for unstable case

Paul E. McKenney (5):
      clocksource: Retry clock read if long delays detected
      clocksource: Check per-CPU clock synchronization when marked unstable
      clocksource: Limit number of CPUs checked for clock synchronization
      clocksource: Reduce clocksource-skew threshold for TSC
      clocksource: Provide kernel module to test clocksource watchdog

 Documentation/admin-guide/kernel-parameters.txt |  22 +++
 arch/x86/kernel/tsc.c                           |   4 +-
 include/linux/clocksource.h                     |   8 +-
 kernel/time/Makefile                            |   1 +
 kernel/time/clocksource-wdtest.c                | 202 +++++++++++++++++++++
 kernel/time/clocksource.c                       | 226 ++++++++++++++++++++++--
 kernel/time/jiffies.c                           |  15 +-
 lib/Kconfig.debug                               |  12 ++
 8 files changed, 468 insertions(+), 22 deletions(-)
 create mode 100644 kernel/time/clocksource-wdtest.c