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Use CLOCK_MONOTONIC for background thread sleep to prevent clock rollback stalls

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background_thread_sleep() computed the pthread_cond_timedwait deadline
from gettimeofday() (CLOCK_REALTIME). If the wall clock jumps backward
(NTP correction, VM snapshot, leap second), the deadline ends up far in
the future and the background thread stops purging dirty pages -
potentially for hours or days. This was observed in production where
NTP corrections caused unbounded RSS growth due to the background
thread stalling.

Switch deadline computation to nstime_init_update() (which already
handles the monotonic vs realtime fallback), and set CLOCK_MONOTONIC
on the condvar via pthread_condattr_setclock at both init sites
(background_thread_boot1 and background_thread_postfork_child), with
fallback to CLOCK_REALTIME if condattr init/setclock fails.
This commit is contained in:
Antonio Andelic 2026-07-01 15:51:15 +02:00 • committed by Guangli Dai
parent fb5499aa9c
commit ebacec3a1a
1 changed files with 41 additions and 6 deletions
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47
src/background_thread.c
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@ -199,11 +199,8 @@ background_thread_sleep(
}
info->npages_to_purge_new = 0;
struct timeval tv;
/* Specific clock required by timedwait. */
gettimeofday(&tv, NULL);
nstime_t before_sleep;
nstime_init2(&before_sleep, tv.tv_sec, tv.tv_usec * 1000);
nstime_init_update(&before_sleep);
int ret;
if (interval == BACKGROUND_THREAD_INDEFINITE_SLEEP) {
@ -235,9 +232,8 @@ background_thread_sleep(
assert(ret == ETIMEDOUT || ret == 0);
}
if (config_stats) {
gettimeofday(&tv, NULL);
nstime_t after_sleep;
nstime_init2(&after_sleep, tv.tv_sec, tv.tv_usec * 1000);
nstime_init_update(&after_sleep);
if (nstime_compare(&after_sleep, &before_sleep) > 0) {
nstime_subtract(&after_sleep, &before_sleep);
nstime_add(&info->tot_sleep_time, &after_sleep);
@ -751,7 +747,27 @@ background_thread_postfork_child(tsdn_t *tsdn) {
background_thread_info_t *info = &background_thread_info[i];
malloc_mutex_lock(tsdn, &info->mtx);
info->state = background_thread_stopped;
#if defined(JEMALLOC_HAVE_CLOCK_MONOTONIC_COARSE) || defined(JEMALLOC_HAVE_CLOCK_MONOTONIC)
int ret;
pthread_condattr_t cond_attr;
if (pthread_condattr_init(&cond_attr)) {
ret = pthread_cond_init(&info->cond, NULL);
} else if (pthread_condattr_setclock(&cond_attr,
CLOCK_MONOTONIC)) {
/*
* Fall back to default (CLOCK_REALTIME)
* attributes if setclock fails.
*/
pthread_condattr_destroy(&cond_attr);
ret = pthread_cond_init(&info->cond, NULL);
} else {
ret = pthread_cond_init(&info->cond,
&cond_attr);
pthread_condattr_destroy(&cond_attr);
}
#else
int ret = pthread_cond_init(&info->cond, NULL);
#endif
assert(ret == 0);
background_thread_info_init(tsdn, info);
malloc_mutex_unlock(tsdn, &info->mtx);
@ -869,9 +885,28 @@ background_thread_boot1(tsdn_t *tsdn, base_t *base) {
malloc_mutex_address_ordered)) {
return true;
}
#if defined(JEMALLOC_HAVE_CLOCK_MONOTONIC_COARSE) || defined(JEMALLOC_HAVE_CLOCK_MONOTONIC)
{
pthread_condattr_t cond_attr;
if (pthread_condattr_init(&cond_attr)) {
return true;
}
if (pthread_condattr_setclock(&cond_attr,
CLOCK_MONOTONIC)) {
pthread_condattr_destroy(&cond_attr);
return true;
}
if (pthread_cond_init(&info->cond, &cond_attr)) {
pthread_condattr_destroy(&cond_attr);
return true;
}
pthread_condattr_destroy(&cond_attr);
}
#else
if (pthread_cond_init(&info->cond, NULL)) {
return true;
}
#endif
malloc_mutex_lock(tsdn, &info->mtx);
info->state = background_thread_stopped;
background_thread_info_init(tsdn, info);