Merge tag 'clocksource.2023.02.06b' of git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu into timers/core

Pull clocksource watchdog changes from Paul McKenney:

     o	Improvements to clocksource-watchdog console messages.

     o	Loosening of the clocksource-watchdog skew criteria to match
     	those of NTP (500 parts per million, relaxed from 400 parts
     	per million).  If it is good enough for NTP, it is good enough
     	for the clocksource watchdog.

     o	Suspend clocksource-watchdog checking temporarily when high
     	memory latencies are detected.	This avoids the false-positive
     	clock-skew events that have been seen on production systems
     	running memory-intensive workloads.

     o	On systems where the TSC is deemed trustworthy, use it as the
     	watchdog timesource, but only when specifically requested using
     	the tsc=watchdog kernel boot parameter.  This permits clock-skew
     	events to be detected, but avoids forcing workloads to use the
     	slow HPET and ACPI PM timers.  These last two timers are slow
     	enough to cause systems to be needlessly marked bad on the one
     	hand, and real skew does sometimes happen on production systems
     	running production workloads on the other.  And sometimes it is
     	the fault of the TSC, or at least of the firmware that told the
     	kernel to program the TSC with the wrong frequency.

     o	Add a tsc=revalidate kernel boot parameter to allow the kernel
     	to diagnose cases where the TSC hardware works fine, but was told
     	by firmware to tick at the wrong frequency.  Such cases are rare,
     	but they really have happened on production systems.

Link: https://lore.kernel.org/r/20230210193640.GA3325193@paulmck-ThinkPad-P17-Gen-1

1 files changed, 10 insertions, 0 deletions

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 6cfa6e3996cf..7b4df6d89d3c 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -6369,6 +6369,16 @@
 			in situations with strict latency requirements (where
 			interruptions from clocksource watchdog are not
 			acceptable).
+			[x86] recalibrate: force recalibration against a HW timer
+			(HPET or PM timer) on systems whose TSC frequency was
+			obtained from HW or FW using either an MSR or CPUID(0x15).
+			Warn if the difference is more than 500 ppm.
+			[x86] watchdog: Use TSC as the watchdog clocksource with
+			which to check other HW timers (HPET or PM timer), but
+			only on systems where TSC has been deemed trustworthy.
+			This will be suppressed by an earlier tsc=nowatchdog and
+			can be overridden by a later tsc=nowatchdog.  A console
+			message will flag any such suppression or overriding.
 
 	tsc_early_khz=  [X86] Skip early TSC calibration and use the given
 			value instead. Useful when the early TSC frequency discovery