clocksource: Limit number of CPUs checked for clock synchronization

Currently, if skew is detected on a clock marked CLOCK_SOURCE_VERIFY_PERCPU,
that clock is checked on all CPUs.  This is thorough, but might not be
what you want on a system with a few tens of CPUs, let alone a few hundred
of them.

Therefore, by default check only up to eight randomly chosen CPUs.  Also
provide a new clocksource.verify_n_cpus kernel boot parameter.  A value of
-1 says to check all of the CPUs, and a non-negative value says to randomly
select that number of CPUs, without concern about selecting the same CPU
multiple times.  However, make use of a cpumask so that a given CPU will be
checked at most once.

Suggested-by: Thomas Gleixner <tglx@linutronix.de> # For verify_n_cpus=1.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Feng Tang <feng.tang@intel.com>
Link: https://lore.kernel.org/r/20210527190124.440372-3-paulmck@kernel.org

1 files changed, 10 insertions, 0 deletions

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 995deccc28bc..9ec9ea1a51f2 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -587,6 +587,16 @@
 			unstable.  Defaults to three retries, that is,
 			four attempts to read the clock under test.
 
+	clocksource.verify_n_cpus= [KNL]
+			Limit the number of CPUs checked for clocksources
+			marked with CLOCK_SOURCE_VERIFY_PERCPU that
+			are marked unstable due to excessive skew.
+			A negative value says to check all CPUs, while
+			zero says not to check any.  Values larger than
+			nr_cpu_ids are silently truncated to nr_cpu_ids.
+			The actual CPUs are chosen randomly, with
+			no replacement if the same CPU is chosen twice.
+
 	clearcpuid=BITNUM[,BITNUM...] [X86]
 			Disable CPUID feature X for the kernel. See
 			arch/x86/include/asm/cpufeatures.h for the valid bit