[PATCH v13 clocksource 4/5] clocksource: Reduce clocksource-skew threshold for TSC

[Paul E. McKenney]

Currently, WATCHDOG_THRESHOLD is set to detect a 62.5-millisecond skew in
a 500-millisecond WATCHDOG_INTERVAL.  This requires that clocks be skewed
by more than 12.5% in order to be marked unstable.  Except that a clock
that is skewed by that much is probably destroying unsuspecting software
right and left.  And given that there are now checks for false-positive
skews due to delays between reading the two clocks, it should be possible
to greatly decrease WATCHDOG_THRESHOLD, at least for fine-grained clocks
such as TSC.

Therefore, add a new uncertainty_margin field to the clocksource
structure that contains the maximum uncertainty in nanoseconds for
the corresponding clock.  This field may be initialized manually,
as it is for clocksource_tsc_early and clocksource_jiffies, which
is copied to refined_jiffies.  If the field is not initialized
manually, it will be computed at clock-registry time as the period
of the clock in question based on the scale and freq parameters to
__clocksource_update_freq_scale() function.  If either of those two
parameters are zero, the tens-of-milliseconds WATCHDOG_THRESHOLD is
used as a cowardly alternative to dividing by zero.  No matter how the
uncertainty_margin field is calculated, it is bounded below by twice
WATCHDOG_MAX_SKEW, that is, by 100 microseconds.

Note that manually initialized uncertainty_margin fields are not adjusted,
but there is a WARN_ON_ONCE() that triggers if any such field is less than
twice WATCHDOG_MAX_SKEW.  This WARN_ON_ONCE() is intended to discourage
production use of the one-nanosecond uncertainty_margin values that are
used to test the clock-skew code itself.

The actual clock-skew check uses the sum of the uncertainty_margin fields
of the two clocksource structures being compared.  Integer overflow is
avoided because the largest computed value of the uncertainty_margin
fields is one billion (10^9), and double that value fits into an
unsigned int.  However, if someone manually specifies (say) UINT_MAX,
they will get what they deserve.

Note that the refined_jiffies uncertainty_margin field is initialized to
TICK_NSEC, which means that skew checks involving this clocksource will
be sufficently forgiving.  In a similar vein, the clocksource_tsc_early
uncertainty_margin field is initialized to 32*NSEC_PER_MSEC, which
replicates the current behavior and allows custom setting if needed
in order to address the rare skews detected for this clocksource in
current mainline.

Link: https://lore.kernel.org/lkml/202104291438.PuHsxRkl-lkp@xxxxxxxxx/
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@xxxxxxxxxxxxx>
Cc: John Stultz <john.stultz@xxxxxxxxxx>
Cc: Stephen Boyd <sboyd@xxxxxxxxxx>
Cc: Jonathan Corbet <corbet@xxxxxxx>
Cc: Mark Rutland <Mark.Rutland@xxxxxxx>
Cc: Marc Zyngier <maz@xxxxxxxxxx>
Cc: Andi Kleen <ak@xxxxxxxxxxxxxxx>
Cc: Xing Zhengjun <zhengjun.xing@xxxxxxxxxxxxxxx>
Cc: Feng Tang <feng.tang@xxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
---
 arch/x86/kernel/tsc.c       |  1 +
 include/linux/clocksource.h |  3 +++
 kernel/time/clocksource.c   | 48 +++++++++++++++++++++++++++++--------
 kernel/time/jiffies.c       | 15 ++++++------
 4 files changed, 50 insertions(+), 17 deletions(-)

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 56289170753c..6e11c9619437 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,
+	.uncertainty_margin	= 32 * 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..8f87c1a6f323 100644
--- a/include/linux/clocksource.h
+++ b/include/linux/clocksource.h
@@ -42,6 +42,8 @@ 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%)
+ * @uncertainty_margin:	Maximum uncertainty in nanoseconds per half second.
+ *			Zero says to use default WATCHDOG_THRESHOLD.
  * @archdata:		Optional arch-specific data
  * @max_cycles:		Maximum safe cycle value which won't overflow on
  *			multiplication
@@ -93,6 +95,7 @@ struct clocksource {
 	u32			shift;
 	u64			max_idle_ns;
 	u32			maxadj;
+	u32			uncertainty_margin;
 #ifdef CONFIG_ARCH_CLOCKSOURCE_DATA
 	struct arch_clocksource_data archdata;
 #endif
diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index 8a7d704aa46b..a6de23800b65 100644
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -95,6 +95,20 @@ static char override_name[CS_NAME_LEN];
 static int finished_booting;
 static u64 suspend_start;
 
+/*
+ * Threshold: 0.0312s, when doubled: 0.0625s.
+ * Also a default for cs->uncertainty_margin when registering clocks.
+ */
+#define WATCHDOG_THRESHOLD (NSEC_PER_SEC >> 5)
+
+/*
+ * Maximum permissible delay between two readouts of the watchdog
+ * clocksource surrounding a read of the clocksource being validated.
+ * This delay could be due to SMIs, NMIs, or to VCPU preemptions.  Used as
+ * a lower bound for cs->uncertainty_margin values when registering clocks.
+ */
+#define WATCHDOG_MAX_SKEW (50 * NSEC_PER_USEC)
+
 #ifdef CONFIG_CLOCKSOURCE_WATCHDOG
 static void clocksource_watchdog_work(struct work_struct *work);
 static void clocksource_select(void);
@@ -121,17 +135,9 @@ static int clocksource_watchdog_kthread(void *data);
 static void __clocksource_change_rating(struct clocksource *cs, int rating);
 
 /*
- * Interval: 0.5sec Threshold: 0.0625s
+ * Interval: 0.5sec.
  */
 #define WATCHDOG_INTERVAL (HZ >> 1)
-#define WATCHDOG_THRESHOLD (NSEC_PER_SEC >> 4)
-
-/*
- * Maximum permissible delay between two readouts of the watchdog
- * clocksource surrounding a read of the clocksource being validated.
- * This delay could be due to SMIs, NMIs, or to VCPU preemptions.
- */
-#define WATCHDOG_MAX_SKEW (100 * NSEC_PER_USEC)
 
 static void clocksource_watchdog_work(struct work_struct *work)
 {
@@ -347,6 +353,7 @@ 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)
@@ -393,7 +400,8 @@ static void clocksource_watchdog(struct timer_list *unused)
 			continue;
 
 		/* Check the deviation from the watchdog clocksource. */
-		if (abs(cs_nsec - wd_nsec) > WATCHDOG_THRESHOLD) {
+		md = cs->uncertainty_margin + watchdog->uncertainty_margin;
+		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",
@@ -1046,6 +1054,26 @@ void __clocksource_update_freq_scale(struct clocksource *cs, u32 scale, u32 freq
 		clocks_calc_mult_shift(&cs->mult, &cs->shift, freq,
 				       NSEC_PER_SEC / scale, sec * scale);
 	}
+
+	/*
+	 * If the uncertainty margin is not specified, calculate it.
+	 * If both scale and freq are non-zero, calculate the clock
+	 * period, but bound below at 2*WATCHDOG_MAX_SKEW.  However,
+	 * if either of scale or freq is zero, be very conservative and
+	 * take the tens-of-milliseconds WATCHDOG_THRESHOLD value for the
+	 * uncertainty margin.  Allow stupidly small uncertainty margins
+	 * to be specified by the caller for testing purposes, but warn
+	 * to discourage production use of this capability.
+	 */
+	if (scale && freq && !cs->uncertainty_margin) {
+		cs->uncertainty_margin = NSEC_PER_SEC / (scale * freq);
+		if (cs->uncertainty_margin < 2 * WATCHDOG_MAX_SKEW)
+			cs->uncertainty_margin = 2 * WATCHDOG_MAX_SKEW;
+	} else if (!cs->uncertainty_margin) {
+		cs->uncertainty_margin = WATCHDOG_THRESHOLD;
+	}
+	WARN_ON_ONCE(cs->uncertainty_margin < 2 * WATCHDOG_MAX_SKEW);
+
 	/*
 	 * Ensure clocksources that have large 'mult' values don't overflow
 	 * when adjusted.
diff --git a/kernel/time/jiffies.c b/kernel/time/jiffies.c
index a5cffe2a1770..165b85bcdf29 100644
--- a/kernel/time/jiffies.c
+++ b/kernel/time/jiffies.c
@@ -49,13 +49,14 @@ static u64 jiffies_read(struct clocksource *cs)
  * for "tick-less" systems.
  */
 static struct clocksource clocksource_jiffies = {
-	.name		= "jiffies",
-	.rating		= 1, /* lowest valid rating*/
-	.read		= jiffies_read,
-	.mask		= CLOCKSOURCE_MASK(32),
-	.mult		= TICK_NSEC << JIFFIES_SHIFT, /* details above */
-	.shift		= JIFFIES_SHIFT,
-	.max_cycles	= 10,
+	.name			= "jiffies",
+	.rating			= 1, /* lowest valid rating*/
+	.uncertainty_margin	= TICK_NSEC,
+	.read			= jiffies_read,
+	.mask			= CLOCKSOURCE_MASK(32),
+	.mult			= TICK_NSEC << JIFFIES_SHIFT, /* details above */
+	.shift			= JIFFIES_SHIFT,
+	.max_cycles		= 10,
 };
 
 __cacheline_aligned_in_smp DEFINE_RAW_SPINLOCK(jiffies_lock);
-- 
2.31.1.189.g2e36527f23