time: Prevent early expiry of hrtimers[CLOCK_REALTIME] at the leap second edge

Message ID 1474341440-16218-2-git-send-email-yadi.hu@windriver.com
State New
Headers show

Commit Message

Yadi Hu Sept. 20, 2016, 3:17 a.m.
From: John Stultz <john.stultz@linaro.org>


Currently, leapsecond adjustments are done at tick time. As a result,
the leapsecond was applied at the first timer tick *after* the
leapsecond (~1-10ms late depending on HZ), rather then exactly on the
second edge.

This was in part historical from back when we were always tick based,
but correcting this since has been avoided since it adds extra
conditional checks in the gettime fastpath, which has performance
overhead.

However, it was recently pointed out that ABS_TIME CLOCK_REALTIME
timers set for right after the leapsecond could fire a second early,
since some timers may be expired before we trigger the timekeeping
timer, which then applies the leapsecond.

This isn't quite as bad as it sounds, since behaviorally it is similar
to what is possible w/ ntpd made leapsecond adjustments done w/o using
the kernel discipline. Where due to latencies, timers may fire just
prior to the settimeofday call. (Also, one should note that all
applications using CLOCK_REALTIME timers should always be careful,
since they are prone to quirks from settimeofday() disturbances.)

However, the purpose of having the kernel do the leap adjustment is to
avoid such latencies, so I think this is worth fixing.

So in order to properly keep those timers from firing a second early,
this patch modifies the ntp and timekeeping logic so that we keep
enough state so that the update_base_offsets_now accessor, which
provides the hrtimer core the current time, can check and apply the
leapsecond adjustment on the second edge. This prevents the hrtimer
core from expiring timers too early.

This patch does not modify any other time read path, so no additional
overhead is incurred. However, this also means that the leap-second
continues to be applied at tick time for all other read-paths.

Apologies to Richard Cochran, who pushed for similar changes years
ago, which I resisted due to the concerns about the performance
overhead.

While I suspect this isn't extremely critical, folks who care about
strict leap-second correctness will likely want to watch
this. Potentially a -stable candidate eventually.

Originally-suggested-by: Richard Cochran <richardcochran@gmail.com>
Reported-by: Daniel Bristot de Oliveira <bristot@redhat.com>
Reported-by: Prarit Bhargava <prarit@redhat.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>

Cc: Richard Cochran <richardcochran@gmail.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Jiri Bohac <jbohac@suse.cz>
Cc: Shuah Khan <shuahkh@osg.samsung.com>
Cc: Ingo Molnar <mingo@kernel.org>
Link:
http://lkml.kernel.org/r/1434063297-28657-4-git-send-email-john.stultz@linaro.org
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

Cc: <stable@vger.kernel.org> # 3.4.x
[Yadi: Move do_adjtimex to timekeeping.c and solve context issues]
Signed-off-by: Hu <yadi.hu@windriver.com>

---
 kernel/time/ntp.c         | 47 +++++++++++++++++++++++++++++++++++++++--------
 kernel/time/timekeeping.c | 37 ++++++++++++++++++++++++++++++++++---
 2 files changed, 73 insertions(+), 11 deletions(-)

-- 
1.8.5.2.233.g932f7e4

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Patch

diff --git a/kernel/time/ntp.c b/kernel/time/ntp.c
index 8b70710..4f5083d 100644
--- a/kernel/time/ntp.c
+++ b/kernel/time/ntp.c
@@ -34,6 +34,7 @@  unsigned long			tick_nsec;
 static u64			tick_length;
 static u64			tick_length_base;
 
+#define	SECS_PER_DAY		86400
 #define MAX_TICKADJ		500LL		/* usecs */
 #define MAX_TICKADJ_SCALED \
 	(((MAX_TICKADJ * NSEC_PER_USEC) << NTP_SCALE_SHIFT) / NTP_INTERVAL_FREQ)
@@ -78,6 +79,9 @@  static long			time_adjust;
 /* constant (boot-param configurable) NTP tick adjustment (upscaled)	*/
 static s64			ntp_tick_adj;
 
+/* second value of the next pending leapsecond, or KTIME_MAX if no leap */
+static s64		ntp_next_leap_sec = KTIME_MAX;
+
 #ifdef CONFIG_NTP_PPS
 
 /*
@@ -354,6 +358,8 @@  void ntp_clear(void)
 	time_maxerror	= NTP_PHASE_LIMIT;
 	time_esterror	= NTP_PHASE_LIMIT;
 
+	ntp_next_leap_sec = KTIME_MAX;
+
 	ntp_update_frequency();
 
 	tick_length	= tick_length_base;
@@ -377,6 +383,22 @@  u64 ntp_tick_length(void)
 	return ret;
 }
 
+/**
+ * ntp_get_next_leap - Returns the next leapsecond in CLOCK_REALTIME ktime_t
+ *
+ * Provides the time of the next leapsecond against CLOCK_REALTIME in
+ * a ktime_t format. Returns KTIME_MAX if no leapsecond is pending.
+ */
+ktime_t ntp_get_next_leap(void)
+{
+	ktime_t ret;
+
+	if ((time_state == TIME_INS) && (time_status & STA_INS))
+		return ktime_set(ntp_next_leap_sec, 0);
+	ret.tv64 = KTIME_MAX;
+	return ret;
+}
+
 
 /*
  * this routine handles the overflow of the microsecond field
@@ -403,15 +425,21 @@  int second_overflow(unsigned long secs)
 	 */
 	switch (time_state) {
 	case TIME_OK:
-		if (time_status & STA_INS)
+		if (time_status & STA_INS) {
 			time_state = TIME_INS;
-		else if (time_status & STA_DEL)
+			ntp_next_leap_sec = secs + SECS_PER_DAY -
+						(secs % SECS_PER_DAY);
+		} else if (time_status & STA_DEL) {
 			time_state = TIME_DEL;
+			ntp_next_leap_sec = secs + SECS_PER_DAY -
+						 ((secs+1) % SECS_PER_DAY);
+		}
 		break;
 	case TIME_INS:
-		if (!(time_status & STA_INS))
+		if (!(time_status & STA_INS)) {
 			time_state = TIME_OK;
-		else if (secs % 86400 == 0) {
+			ntp_next_leap_sec = KTIME_MAX;
+		} else if (secs % SECS_PER_DAY == 0) {
 			leap = -1;
 			time_state = TIME_OOP;
 			time_tai++;
@@ -420,10 +448,12 @@  int second_overflow(unsigned long secs)
 		}
 		break;
 	case TIME_DEL:
-		if (!(time_status & STA_DEL))
+		if (!(time_status & STA_DEL)) {
 			time_state = TIME_OK;
-		else if ((secs + 1) % 86400 == 0) {
+			ntp_next_leap_sec = KTIME_MAX;
+		} else if ((secs + 1) % SECS_PER_DAY == 0) {
 			leap = 1;
+			ntp_next_leap_sec = KTIME_MAX;
 			time_tai--;
 			time_state = TIME_WAIT;
 			printk(KERN_NOTICE
@@ -432,6 +462,7 @@  int second_overflow(unsigned long secs)
 		break;
 	case TIME_OOP:
 		time_state = TIME_WAIT;
+		ntp_next_leap_sec = KTIME_MAX;
 		break;
 
 	case TIME_WAIT:
@@ -549,6 +580,7 @@  static inline void process_adj_status(struct timex *txc, struct timespec *ts)
 	if ((time_status & STA_PLL) && !(txc->status & STA_PLL)) {
 		time_state = TIME_OK;
 		time_status = STA_UNSYNC;
+		ntp_next_leap_sec = KTIME_MAX;
 		/* restart PPS frequency calibration */
 		pps_reset_freq_interval();
 	}
@@ -619,11 +651,10 @@  static inline void process_adjtimex_modes(struct timex *txc, struct timespec *ts
  * adjtimex mainly allows reading (and writing, if superuser) of
  * kernel time-keeping variables. used by xntpd.
  */
-int do_adjtimex(struct timex *txc)
+int __do_adjtimex(struct timex *txc)
 {
 	struct timespec ts;
 	int result;
-
 	/* Validate the data before disabling interrupts */
 	if (txc->modes & ADJ_ADJTIME) {
 		/* singleshot must not be used with any other mode bits */
diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index b0bb93e..60ef9db 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -26,6 +26,8 @@ 
 #include <trace/events/timer.h>
 DEFINE_TRACE(timer_update_time);
 
+extern ktime_t ntp_get_next_leap(void);
+extern int __do_adjtimex(struct timex *);
 
 /* Structure holding internal timekeeping values. */
 struct timekeeper {
@@ -35,7 +37,8 @@  struct timekeeper {
 	u32	mult;
 	/* The shift value of the current clocksource. */
 	int	shift;
-
+	/* CLOCK_MONOTONIC time value of a pending leap-second*/
+	ktime_t			next_leap_ktime;
 	/* Number of clock cycles in one NTP interval. */
 	cycle_t cycle_interval;
 	/* Number of clock shifted nano seconds in one NTP interval. */
@@ -198,6 +201,17 @@  static void update_rt_offset(void)
 	timekeeper.offs_real = timespec_to_ktime(tmp);
 }
 
+/*
+ *   tk_update_leap_state - helper to update the next_leap_ktime
+ */
+static inline void tk_update_leap_state(struct timekeeper *tk)
+{
+	tk->next_leap_ktime = ntp_get_next_leap();
+	if (tk->next_leap_ktime.tv64 != KTIME_MAX)
+		/* Convert to monotonic time */
+		tk->next_leap_ktime = ktime_sub(tk->next_leap_ktime, tk->offs_real);
+}
+
 /* must hold write on timekeeper.lock */
 static void timekeeping_update(bool clearntp)
 {
@@ -205,6 +219,7 @@  static void timekeeping_update(bool clearntp)
 		timekeeper.ntp_error = 0;
 		ntp_clear();
 	}
+	tk_update_leap_state(&timekeeper);
 	update_rt_offset();
 	update_vsyscall(&timekeeper.xtime, &timekeeper.wall_to_monotonic,
 			 timekeeper.clock, timekeeper.mult);
@@ -1346,10 +1361,16 @@  ktime_t ktime_get_update_offsets(ktime_t *offs_real, ktime_t *offs_boot)
 
 		*offs_real = timekeeper.offs_real;
 		*offs_boot = timekeeper.offs_boot;
+
+		now = ktime_add_ns(ktime_set(secs, 0), nsecs);
+		now = ktime_sub(now, *offs_real);
+
+		/* Handle leapsecond insertion adjustments */
+		if (unlikely(now.tv64 >= timekeeper.next_leap_ktime.tv64))
+			*offs_real = ktime_sub(timekeeper.offs_real, ktime_set(1, 0));
+
 	} while (read_seqretry(&timekeeper.lock, seq));
 
-	now = ktime_add_ns(ktime_set(secs, 0), nsecs);
-	now = ktime_sub(now, *offs_real);
 	return now;
 }
 #endif
@@ -1371,6 +1392,16 @@  ktime_t ktime_get_monotonic_offset(void)
 }
 EXPORT_SYMBOL_GPL(ktime_get_monotonic_offset);
 
+/*
+ * do_adjtimex() - Accessor function to NTP __do_adjtimex function
+ */
+int do_adjtimex(struct timex *txc)
+{
+	int ret;
+	ret = __do_adjtimex(txc);
+	tk_update_leap_state(&timekeeper);
+	return ret;
+}
 
 /**
  * xtime_update() - advances the timekeeping infrastructure