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authorJohn Stultz <john.stultz@linaro.org>2015-06-11 15:54:55 -0700
committerThomas Gleixner <tglx@linutronix.de>2015-06-12 11:15:49 +0200
commit833f32d763028c1bb371c64f457788b933773b3e (patch)
treee49045ff3592b68bbce6c155375092b81eb5abed /kernel/time/ntp.c
parent90bf361ceae28dee50a584c3dd4c1a96178d982c (diff)
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time: Prevent early expiry of hrtimers[CLOCK_REALTIME] at the leap second edge
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>
Diffstat (limited to 'kernel/time/ntp.c')
-rw-r--r--kernel/time/ntp.c42
1 files changed, 35 insertions, 7 deletions
diff --git a/kernel/time/ntp.c b/kernel/time/ntp.c
index 7aa216188450..033743e3647a 100644
--- a/kernel/time/ntp.c
+++ b/kernel/time/ntp.c
@@ -77,6 +77,9 @@ 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 TIME64_MAX if no leap */
+static time64_t ntp_next_leap_sec = TIME64_MAX;
+
#ifdef CONFIG_NTP_PPS
/*
@@ -350,6 +353,7 @@ void ntp_clear(void)
tick_length = tick_length_base;
time_offset = 0;
+ ntp_next_leap_sec = TIME64_MAX;
/* Clear PPS state variables */
pps_clear();
}
@@ -360,6 +364,21 @@ u64 ntp_tick_length(void)
return tick_length;
}
+/**
+ * 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
@@ -383,15 +402,21 @@ 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)) {
+ ntp_next_leap_sec = TIME64_MAX;
time_state = TIME_OK;
- else if (secs % SECS_PER_DAY == 0) {
+ } else if (secs % SECS_PER_DAY == 0) {
leap = -1;
time_state = TIME_OOP;
printk(KERN_NOTICE
@@ -399,19 +424,21 @@ int second_overflow(unsigned long secs)
}
break;
case TIME_DEL:
- if (!(time_status & STA_DEL))
+ if (!(time_status & STA_DEL)) {
+ ntp_next_leap_sec = TIME64_MAX;
time_state = TIME_OK;
- else if ((secs + 1) % SECS_PER_DAY == 0) {
+ } else if ((secs + 1) % SECS_PER_DAY == 0) {
leap = 1;
+ ntp_next_leap_sec = TIME64_MAX;
time_state = TIME_WAIT;
printk(KERN_NOTICE
"Clock: deleting leap second 23:59:59 UTC\n");
}
break;
case TIME_OOP:
+ ntp_next_leap_sec = TIME64_MAX;
time_state = TIME_WAIT;
break;
-
case TIME_WAIT:
if (!(time_status & (STA_INS | STA_DEL)))
time_state = TIME_OK;
@@ -548,6 +575,7 @@ static inline void process_adj_status(struct timex *txc, struct timespec64 *ts)
if ((time_status & STA_PLL) && !(txc->status & STA_PLL)) {
time_state = TIME_OK;
time_status = STA_UNSYNC;
+ ntp_next_leap_sec = TIME64_MAX;
/* restart PPS frequency calibration */
pps_reset_freq_interval();
}