From fa218f1cce6ba40069c8daab8821de7e6be1cdd0 Mon Sep 17 00:00:00 2001 From: Paul E. McKenney Date: Thu, 27 May 2021 12:01:21 -0700 Subject: 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 # For verify_n_cpus=1. Signed-off-by: Paul E. McKenney Signed-off-by: Thomas Gleixner Acked-by: Feng Tang Link: https://lore.kernel.org/r/20210527190124.440372-3-paulmck@kernel.org --- Documentation/admin-guide/kernel-parameters.txt | 10 ++++++++++ 1 file changed, 10 insertions(+) (limited to 'Documentation/admin-guide') 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 -- cgit v1.2.3