[PATCH v10 clocksource 6/7] clocksource: Forgive tsc_early pre-calibration drift

["Paul E. McKenney" <paulmck@kernel.org>]

Because the x86 tsc_early clocksource is given a quick and semi-accurate
calibration (by design!), it might have drift rates well in excess of
the 0.1% limit that is in the process of being adopted.

Therefore, add a max_drift field to the clocksource structure that, when
non-zero, specifies the maximum allowable drift rate in nanoseconds over
a half-second period.  The tsc_early clocksource initializes this to five
miliseconds, which corresponds to the 1% drift rate limit suggested by
Xing Zhengjun.  This max_drift field is intended only for early boot,
so clocksource_watchdog() splats if it encounters a non-zero value in
this field more than 60 seconds after boot, inspired by a suggestion by
Thomas Gleixner.

This was tested by setting the clocksource_tsc ->max_drift field to 1,
which, as expected, resulted in a clock-skew event.

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: Xing Zhengjun <zhengjun.xing@linux.intel.com>
Cc: Feng Tang <feng.tang@intel.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
 arch/x86/kernel/tsc.c       |  1 +
 include/linux/clocksource.h |  5 +++++
 kernel/time/clocksource.c   | 19 ++++++++++++++++++-
 3 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 56289170753c..7192b8950322 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -1127,6 +1127,7 @@ static int tsc_cs_enable(struct clocksource *cs)
 static struct clocksource clocksource_tsc_early = {
 	.name			= "tsc-early",
 	.rating			= 299,
+	.max_drift		= 5 * NSEC_PER_MSEC,
 	.read			= read_tsc,
 	.mask			= CLOCKSOURCE_MASK(64),
 	.flags			= CLOCK_SOURCE_IS_CONTINUOUS |
diff --git a/include/linux/clocksource.h b/include/linux/clocksource.h
index 83a3ebff7456..07d25a158d7a 100644
--- a/include/linux/clocksource.h
+++ b/include/linux/clocksource.h
@@ -42,6 +42,10 @@ struct module;
  * @shift:		Cycle to nanosecond divisor (power of two)
  * @max_idle_ns:	Maximum idle time permitted by the clocksource (nsecs)
  * @maxadj:		Maximum adjustment value to mult (~11%)
+ * @max_drift:		Maximum drift rate in nanoseconds per half second.
+ *			Zero says to use default WATCHDOG_THRESHOLD.
+ *			A non-zero value for ->max_drift more than 60 seconds after boot
+ *			will result in a splat.
  * @archdata:		Optional arch-specific data
  * @max_cycles:		Maximum safe cycle value which won't overflow on
  *			multiplication
@@ -93,6 +97,7 @@ struct clocksource {
 	u32			shift;
 	u64			max_idle_ns;
 	u32			maxadj;
+	u32			max_drift;
 #ifdef CONFIG_ARCH_CLOCKSOURCE_DATA
 	struct arch_clocksource_data archdata;
 #endif
diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index f71f375df544..070b09dab815 100644
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -126,6 +126,7 @@ static void __clocksource_change_rating(struct clocksource *cs, int rating);
  */
 #define WATCHDOG_INTERVAL (HZ >> 1)
 #define WATCHDOG_THRESHOLD (NSEC_PER_SEC >> 4)
+#define WATCHDOG_SYNC_FORGIVENESS (HZ * 60UL)
 
 /*
  * Maximum permissible delay between two readouts of the watchdog
@@ -377,6 +378,7 @@ static void clocksource_watchdog(struct timer_list *unused)
 	int next_cpu, reset_pending;
 	int64_t wd_nsec, cs_nsec;
 	struct clocksource *cs;
+	u32 md;
 
 	spin_lock(&watchdog_lock);
 	if (!watchdog_running)
@@ -423,7 +425,22 @@ static void clocksource_watchdog(struct timer_list *unused)
 			continue;
 
 		/* Check the deviation from the watchdog clocksource. */
-		if (abs(cs_nsec - wd_nsec) > WATCHDOG_THRESHOLD) {
+		if (!cs->max_drift) {
+			md = WATCHDOG_THRESHOLD;
+		} else {
+			static unsigned long first_jiffies;
+			static bool beenhere;
+
+			if (beenhere) {
+				WARN_ON_ONCE(time_after(jiffies,
+							first_jiffies + WATCHDOG_SYNC_FORGIVENESS));
+			} else {
+				beenhere = true;
+				first_jiffies = jiffies;
+			}
+			md = cs->max_drift;
+		}
+		if (abs(cs_nsec - wd_nsec) > md) {
 			pr_warn("timekeeping watchdog on CPU%d: Marking clocksource '%s' as unstable because the skew is too large:\n",
 				smp_processor_id(), cs->name);
 			pr_warn("                      '%s' wd_now: %llx wd_last: %llx mask: %llx\n",
-- 
2.31.1.189.g2e36527f23