[PATCH v14 clocksource 3/6] clocksource: Limit number of CPUs checked for clock synchronization

Paul E. McKenney
2021-05-11T23:35:00Z

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.

Link: https://lore.kernel.org/lkml/202104291438.PuHsxRkl-lkp@intel.com/
Link: https://lore.kernel.org/lkml/20210429140440.GT975577@paulmck-ThinkPad-P17-Gen-1
Link: https://lore.kernel.org/lkml/20210425224540.GA1312438@paulmck-ThinkPad-P17-Gen-1/
Link: https://lore.kernel.org/lkml/20210420064934.GE31773@xsang-OptiPlex-9020/
Link: https://lore.kernel.org/lkml/20210106004013.GA11179@paulmck-ThinkPad-P72/
Link: https://lore.kernel.org/lkml/20210414043435.GA2812539@paulmck-ThinkPad-P17-Gen-1/
Link: https://lore.kernel.org/lkml/20210419045155.GA596058@paulmck-ThinkPad-P17-Gen-1/
Suggested-by: Thomas Gleixner <tglx@linutronix.de> # For verify_n_cpus=1.
Cc: John Stultz <john.stultz@linaro.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Stephen Boyd <sboyd@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Mark Rutland <Mark.Rutland@arm.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Feng Tang <feng.tang@intel.com>
Cc: Xing Zhengjun <zhengjun.xing@linux.intel.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
.../admin-guide/kernel-parameters.txt | 10 +++
kernel/time/clocksource.c | 74 ++++++++++++++++++-
2 files changed, 82 insertions(+), 2 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index c6886a44f679..0ef0ee743f65 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
diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index d994762fcdca..66243da2dadb 100644
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -14,6 +14,8 @@
#include <linux/sched.h> /* for spin_unlock_irq() using preempt_count() m68k */
#include <linux/tick.h>
#include <linux/kthread.h>
+#include <linux/prandom.h>
+#include <linux/cpu.h>

#include "tick-internal.h"
#include "timekeeping_internal.h"
@@ -193,6 +195,8 @@ void clocksource_mark_unstable(struct clocksource *cs)

static ulong max_read_retries = 3;
module_param(max_read_retries, ulong, 0644);
+static int verify_n_cpus = 8;
+module_param(verify_n_cpus, int, 0644);

static bool cs_watchdog_read(struct clocksource *cs, u64 *csnow, u64 *wdnow)
{
@@ -226,6 +230,55 @@ static bool cs_watchdog_read(struct clocksource *cs, u64 *csnow, u64 *wdnow)
static u64 csnow_mid;
static cpumask_t cpus_ahead;
static cpumask_t cpus_behind;
+static cpumask_t cpus_chosen;
+
+static void clocksource_verify_choose_cpus(void)
+{
+ int cpu, i, n = verify_n_cpus;
+
+ if (n < 0) {
+ /* Check all of the CPUs. */
+ cpumask_copy(&cpus_chosen, cpu_online_mask);
+ cpumask_clear_cpu(smp_processor_id(), &cpus_chosen);
+ return;
+ }
+
+ /* If no checking desired, or no other CPU to check, leave. */
+ cpumask_clear(&cpus_chosen);
+ if (n == 0 || num_online_cpus() <= 1)
+ return;
+
+ /* Make sure to select at least one CPU other than the current CPU. */
+ cpu = cpumask_next(-1, cpu_online_mask);
+ if (cpu == smp_processor_id())
+ cpu = cpumask_next(cpu, cpu_online_mask);
+ if (WARN_ON_ONCE(cpu >= nr_cpu_ids))
+ return;
+ cpumask_set_cpu(cpu, &cpus_chosen);
+
+ /* Force a sane value for the boot parameter. */
+ if (n > nr_cpu_ids)
+ n = nr_cpu_ids;
+
+ /*
+ * Randomly select the specified number of CPUs. If the same
+ * CPU is selected multiple times, that CPU is checked only once,
+ * and no replacement CPU is selected. This gracefully handles
+ * situations where verify_n_cpus is greater than the number of
+ * CPUs that are currently online.
+ */
+ for (i = 1; i < n; i++) {
+ cpu = prandom_u32() % nr_cpu_ids;
+ cpu = cpumask_next(cpu - 1, cpu_online_mask);
+ if (cpu >= nr_cpu_ids)
+ cpu = cpumask_next(-1, cpu_online_mask);
+ if (!WARN_ON_ONCE(cpu >= nr_cpu_ids))
+ cpumask_set_cpu(cpu, &cpus_chosen);
+ }
+
+ /* Don't verify ourselves. */
+ cpumask_clear_cpu(smp_processor_id(), &cpus_chosen);
+}

static void clocksource_verify_one_cpu(void *csin)
{
@@ -241,12 +294,22 @@ static void clocksource_verify_percpu(struct clocksource *cs)
int cpu, testcpu;
s64 delta;

+ if (verify_n_cpus == 0)
+ return;
cpumask_clear(&cpus_ahead);
cpumask_clear(&cpus_behind);
+ get_online_cpus();
preempt_disable();
+ clocksource_verify_choose_cpus();
+ if (cpumask_weight(&cpus_chosen) == 0) {
+ preempt_enable();
+ put_online_cpus();
+ pr_warn("Not enough CPUs to check clocksource '%s'.\n", cs->name);
+ return;
+ }
testcpu = smp_processor_id();
- pr_warn("Checking clocksource %s synchronization from CPU %d.\n", cs->name, testcpu);
- for_each_online_cpu(cpu) {
+ pr_warn("Checking clocksource %s synchronization from CPU %d to CPUs %*pbl.\n", cs->name, testcpu, cpumask_pr_args(&cpus_chosen));
+ for_each_cpu(cpu, &cpus_chosen) {
if (cpu == testcpu)
continue;
csnow_begin = cs->read(cs);
@@ -266,6 +329,7 @@ static void clocksource_verify_percpu(struct clocksource *cs)
cs_nsec_min = cs_nsec;
}
preempt_enable();
+ put_online_cpus();
if (!cpumask_empty(&cpus_ahead))
pr_warn(" CPUs %*pbl ahead of CPU %d for clocksource %s.\n",
cpumask_pr_args(&cpus_ahead), testcpu, cs->name);
@@ -336,6 +400,12 @@ static void clocksource_watchdog(struct timer_list *unused)
watchdog->name, wdnow, wdlast, watchdog->mask);
pr_warn(" '%s' cs_now: %llx cs_last: %llx mask: %llx\n",
cs->name, csnow, cslast, cs->mask);
+ if (curr_clocksource == cs)
+ pr_warn(" '%s' is current clocksource.\n", cs->name);
+ else if (curr_clocksource)
+ pr_warn(" '%s' (not '%s') is current clocksource.\n", curr_clocksource->name, cs->name);
+ else
+ pr_warn(" No current clocksource.\n");
__clocksource_unstable(cs);
continue;
}
--
2.31.1.189.g2e36527f23