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