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SubjectRe: [PATCH 0/3][RFC] Potential fix for leapsecond caused futex issue (v3)
On 07/02/2012 07:16 PM, John Stultz wrote:
> NOTE: Some reports have been of a hard hang right at or before
> the leapsecond. I've not been able to reproduce or diagnose
> this, so this fix does not likely address the reported hard
> hangs (unless they end up being connected to the futex/hrtimer
> issue). Please email lkml and me if you experienced this.

Since as noted above, I've seen some sporadic reports of hard hangs.
Some seem connected to the hrtimer problem, where ksoftirq seems to go
crazy and cause nmi watchdog lockups, but others are less clear.

I wanted to try to provide a way to stress both the kernel's leapsecond
code as well as provide a way for folks to be able to test their
application's robustness in the face of leapsecond inconsistencies.

Attached is my first attempt at such a test.

It is designed to be run on a server, where it will schedule a
leapsecond every day at midnight GMT. So every day, while it runs, the
server will see a leapsecond. This allows the the leap second, as well
as any suspected timer related lockups that might happen when the
leapsecond is scheduled to be stressed.

The test also outputs time samples right before, during and after the
leapsecond is applied, so you can watch it happen.

Also since once a day is a fairly low frequency, if you pass a "-s" to
the test, it will jump the system time forward to 10 seconds right
before the scheduled leapsecond for that day. Allowing a leapsecond to
occur every ~13 seconds. This mode may cause application disruption, as
it also causes the system to advance a day every ~13 seconds.

The test additionally will note if it observes the hrtimer early
expiration problem that was widely seen over the weekend.

Hopefully this will provide a mechanism to test and maintain the
kernel's correct behaviour for these rare events, as well as allowing
folks to get more comfortable with leapsecond behaviour and test how it
might impact their applications.

If anyone who observed a hard hang is able to use this to reproduce the
problem, I'd greatly like to hear about it.

Build instructions are in the test file.

/* Leap second stress test
* by: john stultz (
* (C) Copyright IBM 2012
* Licensed under the GPLv2
* This test signals the kernel to insert a leap second
* every day at midnight GMT. This allows for stessing the
* kernel's leap-second behavior, as well as how well applications
* handle the leap-second discontinuity.
* Usage: leap-a-day [-s]
* Options:
* -s: Each iteration, set the date to 10 seconds before midnight GMT.
* This speeds up the number of leapsecond transitions tested,
* but because it calls settimeofday frequently, advancing the
* time by 24 hours every ~16 seconds, it may cause application
* disruption.
* Other notes: Disabling NTP prior to running this is advised, as the two
* may conflict in thier commands to the kernel.
* To build:
* $ gcc leap-a-day.c -o leap-a-day -lrt

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <sys/time.h>
#include <sys/timex.h>
#include <string.h>
#include <signal.h>

#define NSEC_PER_SEC 1000000000ULL

/* returns 1 if a <= b, 0 otherwise */
static inline int in_order(struct timespec a, struct timespec b)
if(a.tv_sec < b.tv_sec)
return 1;
if(a.tv_sec > b.tv_sec)
return 0;
if(a.tv_nsec > b.tv_nsec)
return 0;
return 1;

struct timespec timespec_add(struct timespec ts, unsigned long long ns)
ts.tv_nsec += ns;
while(ts.tv_nsec >= NSEC_PER_SEC) {
ts.tv_nsec -= NSEC_PER_SEC;
return ts;

char* time_state_str(int state)
switch (state) {
case TIME_OK: return "TIME_OK";
case TIME_INS: return "TIME_INS";
case TIME_DEL: return "TIME_DEL";
case TIME_OOP: return "TIME_OOP";
case TIME_WAIT: return "TIME_WAIT";
case TIME_BAD: return "TIME_BAD";
return "ERROR";

/* clear NTP time_status & time_state */
void clear_time_state(void)
struct timex tx;
int ret;

* XXX - The fact we have to call this twice seems
* to point to a slight issue in the kernel's ntp state
* managment. Needs to be investigated further.

tx.modes = ADJ_STATUS;
tx.status = STA_PLL;
ret = adjtimex(&tx);

tx.modes = ADJ_STATUS;
tx.status = 0;
ret = adjtimex(&tx);

/* Make sure we cleanup on ctrl-c */
void handler(int unused)

/* Test for known hrtimer failure */
void test_hrtimer_failure(void)
struct timespec now, target;

clock_gettime(CLOCK_REALTIME, &now);
target = timespec_add(now, NSEC_PER_SEC/2);
clock_nanosleep(CLOCK_REALTIME, TIMER_ABSTIME, &target, NULL);
clock_gettime(CLOCK_REALTIME, &now);

if (!in_order(target, now)) {
printf("Note: hrtimer early expiration failure observed.\n");


int main(int argc, char** argv)
struct timeval tv;
struct timex tx;
struct timespec ts;
int settime = 0;

signal(SIGINT, handler);
signal(SIGKILL, handler);
printf("This runs continuously. Press ctrl-c to stop\n");

/* Process arguments */
if (argc > 1) {
if (!strncmp(argv[1], "-s", 2)) {
printf("Setting time to speed up testing\n");
settime = 1;
} else {
printf("Usage: %s [-s]\n", argv[0]);
printf(" -s: Set time to right before leap second each iteration\n");

while (1) {
int ret;
time_t now, next_leap;
/* Get the current time */
gettimeofday(&tv, NULL);

/* Calculate the next possible leap second 23:59:60 GMT */
tv.tv_sec += 86400 - (tv.tv_sec % 86400);
next_leap = ts.tv_sec = tv.tv_sec;

if (settime) {
tv.tv_sec -= 10;
settimeofday(&tv, NULL);
printf("Setting time to %s", ctime(&ts.tv_sec));

/* Reset NTP time state */

/* Set the leap second insert flag */
tx.modes = ADJ_STATUS;
tx.status = STA_INS;
ret = adjtimex(&tx);
if (ret) {
printf("Error: Problem setting STA_INS!: %s\n",
return -1;

/* Validate STA_INS was set */
ret = adjtimex(&tx);
if (tx.status != STA_INS) {
printf("Error: STA_INS not set!: %s\n",
return -1;

printf("Scheduling leap second for %s", ctime(&next_leap));

/* Wake up 3 seconds before leap */
ts.tv_sec -= 3;
if(clock_nanosleep(CLOCK_REALTIME, TIMER_ABSTIME, &ts, NULL))
printf("Something woke us up, returning to sleep\n");

/* Validate STA_INS is still set */
ret = adjtimex(&tx);
if (tx.status != STA_INS) {
printf("Something cleared STA_INS, setting it again.\n");
tx.modes = ADJ_STATUS;
tx.status = STA_INS;
ret = adjtimex(&tx);

/* Check adjtimex output every half second */
now = tx.time.tv_sec;
while (now < next_leap+2) {
char buf[26];
ret = adjtimex(&tx);

ctime_r(&tx.time.tv_sec, buf);
buf[strlen(buf)-1] = 0; /*remove trailing\n */

printf("%s + %6ld us\t%s\n",
now = tx.time.tv_sec;
/* Sleep for another half second */
ts.tv_sec = 0;
ts.tv_nsec = NSEC_PER_SEC/2;
clock_nanosleep(CLOCK_MONOTONIC, 0, &ts, NULL);

/* Note if kernel has known hrtimer failure */

printf("Leap complete\n\n");

return 0;
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