clocksource: Check per-CPU clock synchronization when marked unstable

Some sorts of per-CPU clock sources have a history of going out of
synchronization with each other.  However, this problem has purportedy been
solved in the past ten years.  Except that it is all too possible that the
problem has instead simply been made less likely, which might mean that
some of the occasional "Marking clocksource 'tsc' as unstable" messages
might be due to desynchronization.  How would anyone know?

Therefore apply CPU-to-CPU synchronization checking to newly unstable
clocksource that are marked with the new CLOCK_SOURCE_VERIFY_PERCPU flag.
Lists of desynchronized CPUs are printed, with the caveat that if it
is the reporting CPU that is itself desynchronized, it will appear that
all the other clocks are wrong.  Just like in real life.
Reported-by: Chris Mason <clm@fb.com>
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/20210527190124.440372-2-paulmck@kernel.org

arch/x86/kernel/tsc.c

@@ -1152,7 +1152,8 @@ static struct clocksource clocksource_tsc = {
 	.mask			= CLOCKSOURCE_MASK(64),
 	.flags			= CLOCK_SOURCE_IS_CONTINUOUS |
 				  CLOCK_SOURCE_VALID_FOR_HRES |
-				  CLOCK_SOURCE_MUST_VERIFY,
+				  CLOCK_SOURCE_MUST_VERIFY |
+				  CLOCK_SOURCE_VERIFY_PERCPU,
 	.vdso_clock_mode	= VDSO_CLOCKMODE_TSC,
 	.enable			= tsc_cs_enable,
 	.resume			= tsc_resume,

include/linux/clocksource.h

@@ -137,7 +137,7 @@ struct clocksource {
 #define CLOCK_SOURCE_UNSTABLE			0x40
 #define CLOCK_SOURCE_SUSPEND_NONSTOP		0x80
 #define CLOCK_SOURCE_RESELECT			0x100
-
+#define CLOCK_SOURCE_VERIFY_PERCPU		0x200
 /* simplify initialization of mask field */
 #define CLOCKSOURCE_MASK(bits) GENMASK_ULL((bits) - 1, 0)
 

kernel/time/clocksource.c

@@ -224,6 +224,60 @@ static bool cs_watchdog_read(struct clocksource *cs, u64 *csnow, u64 *wdnow)
 	return false;
 }
 
+static u64 csnow_mid;
+static cpumask_t cpus_ahead;
+static cpumask_t cpus_behind;
+
+static void clocksource_verify_one_cpu(void *csin)
+{
+	struct clocksource *cs = (struct clocksource *)csin;
+
+	csnow_mid = cs->read(cs);
+}
+
+static void clocksource_verify_percpu(struct clocksource *cs)
+{
+	int64_t cs_nsec, cs_nsec_max = 0, cs_nsec_min = LLONG_MAX;
+	u64 csnow_begin, csnow_end;
+	int cpu, testcpu;
+	s64 delta;
+
+	cpumask_clear(&cpus_ahead);
+	cpumask_clear(&cpus_behind);
+	preempt_disable();
+	testcpu = smp_processor_id();
+	pr_warn("Checking clocksource %s synchronization from CPU %d.\n", cs->name, testcpu);
+	for_each_online_cpu(cpu) {
+		if (cpu == testcpu)
+			continue;
+		csnow_begin = cs->read(cs);
+		smp_call_function_single(cpu, clocksource_verify_one_cpu, cs, 1);
+		csnow_end = cs->read(cs);
+		delta = (s64)((csnow_mid - csnow_begin) & cs->mask);
+		if (delta < 0)
+			cpumask_set_cpu(cpu, &cpus_behind);
+		delta = (csnow_end - csnow_mid) & cs->mask;
+		if (delta < 0)
+			cpumask_set_cpu(cpu, &cpus_ahead);
+		delta = clocksource_delta(csnow_end, csnow_begin, cs->mask);
+		cs_nsec = clocksource_cyc2ns(delta, cs->mult, cs->shift);
+		if (cs_nsec > cs_nsec_max)
+			cs_nsec_max = cs_nsec;
+		if (cs_nsec < cs_nsec_min)
+			cs_nsec_min = cs_nsec;
+	}
+	preempt_enable();
+	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);
+	if (!cpumask_empty(&cpus_behind))
+		pr_warn("        CPUs %*pbl behind CPU %d for clocksource %s.\n",
+			cpumask_pr_args(&cpus_behind), testcpu, cs->name);
+	if (!cpumask_empty(&cpus_ahead) || !cpumask_empty(&cpus_behind))
+		pr_warn("        CPU %d check durations %lldns - %lldns for clocksource %s.\n",
+			testcpu, cs_nsec_min, cs_nsec_max, cs->name);
+}
+
 static void clocksource_watchdog(struct timer_list *unused)
 {
 	u64 csnow, wdnow, cslast, wdlast, delta;
@@ -448,6 +502,12 @@ static int __clocksource_watchdog_kthread(void)
 	unsigned long flags;
 	int select = 0;
 
+	/* Do any required per-CPU skew verification. */
+	if (curr_clocksource &&
+	    curr_clocksource->flags & CLOCK_SOURCE_UNSTABLE &&
+	    curr_clocksource->flags & CLOCK_SOURCE_VERIFY_PERCPU)
+		clocksource_verify_percpu(curr_clocksource);
+
 	spin_lock_irqsave(&watchdog_lock, flags);
 	list_for_each_entry_safe(cs, tmp, &watchdog_list, wd_list) {
 		if (cs->flags & CLOCK_SOURCE_UNSTABLE) {