[PATCH v14 clocksource 4/6] clocksource: Reduce clocksource-skew threshold for TSC

Paul E. McKenney
2021-05-11T23:35:07Z

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@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>
Cc: John Stultz <john.stultz@linaro.org>
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 | 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 6eb1b097e97e..2e076a459a0c 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -1128,6 +1128,7 @@ 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 7f83d51c0fd7..895203727cb5 100644
--- a/include/linux/clocksource.h
+++ b/include/linux/clocksource.h
@@ -43,6 +43,8 @@ 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
@@ -98,6 +100,7 @@ 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 66243da2dadb..9ebf9931f3d6 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 a492e4da69ba..b3f608c2b936 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