Detect monotonic condvar support at configure time

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.

configure.ac

@@ -2224,6 +2224,43 @@ dnl Check if we have dlsym support.
   if test "x${je_cv_pthread_get_name_np}" = "xyes" ; then
     AC_DEFINE([JEMALLOC_HAVE_PTHREAD_GET_NAME_NP], [ ], [ ])
   fi
+  dnl Runtime-check pthread_condattr_setclock and clock_gettime with
+  dnl CLOCK_MONOTONIC to confirm the kernel actually supports the
+  dnl monotonic clock.
+  AC_CACHE_CHECK([whether pthread_condattr_setclock with CLOCK_MONOTONIC is usable],
+                 [je_cv_pthread_cond_timedwait_monotonic],
+                 AC_RUN_IFELSE([AC_LANG_PROGRAM(
+[[
+#include <pthread.h>
+#include <time.h>
+]], [[
+  pthread_condattr_t attr;
+  pthread_cond_t cond;
+  struct timespec ts;
+  int ret = pthread_condattr_init(&attr);
+  if (ret != 0) {
+    return 1;
+  }
+  ret = pthread_condattr_setclock(&attr, CLOCK_MONOTONIC);
+  if (ret != 0) {
+    pthread_condattr_destroy(&attr);
+    return 1;
+  }
+  ret = pthread_cond_init(&cond, &attr);
+  pthread_condattr_destroy(&attr);
+  if (ret != 0) {
+    return 1;
+  }
+  ret = clock_gettime(CLOCK_MONOTONIC, &ts);
+  pthread_cond_destroy(&cond);
+  return ret != 0;
+]])],
+                 [je_cv_pthread_cond_timedwait_monotonic=yes],
+                 [je_cv_pthread_cond_timedwait_monotonic=no],
+                 [je_cv_pthread_cond_timedwait_monotonic=no]))
+  if test "x${je_cv_pthread_cond_timedwait_monotonic}" = "xyes" ; then
+    AC_DEFINE([JEMALLOC_HAVE_PTHREAD_COND_TIMEDWAIT_MONOTONIC], [ ], [ ])
+  fi
 fi
 
 JE_APPEND_VS(CPPFLAGS, -D_REENTRANT)

include/jemalloc/internal/jemalloc_internal_defs.h.in

@@ -97,6 +97,9 @@
 /* Defined if pthread_get_name_np(3) is available. */
 #undef JEMALLOC_HAVE_PTHREAD_GET_NAME_NP
 
+/* Defined if pthread_condattr_setclock(..., CLOCK_MONOTONIC) is available. */
+#undef JEMALLOC_HAVE_PTHREAD_COND_TIMEDWAIT_MONOTONIC
+
 /*
  * Defined if clock_gettime(CLOCK_MONOTONIC_COARSE, ...) is available.
  */

src/background_thread.c

@@ -191,6 +191,50 @@ background_thread_cond_wait(
 	return ret;
 }
 
+static int
+background_thread_cond_init(pthread_cond_t *cond) {
+#ifdef JEMALLOC_HAVE_PTHREAD_COND_TIMEDWAIT_MONOTONIC
+	pthread_condattr_t cond_attr;
+	int ret = pthread_condattr_init(&cond_attr);
+	if (ret != 0) {
+		return ret;
+	}
+	ret = pthread_condattr_setclock(&cond_attr, CLOCK_MONOTONIC);
+	if (ret != 0) {
+		pthread_condattr_destroy(&cond_attr);
+		return ret;
+	}
+	ret = pthread_cond_init(cond, &cond_attr);
+	pthread_condattr_destroy(&cond_attr);
+	return ret;
+#else
+	return pthread_cond_init(cond, NULL);
+#endif
+}
+
+/*
+ * Fill in the absolute deadline for pthread_cond_timedwait.  The clock read
+ * here MUST match the clock the condvar was initialized with in
+ * background_thread_cond_init, otherwise the deadline is interpreted against
+ * the wrong epoch.
+ */
+static void
+background_thread_wakeup_ts_init(struct timespec *ts, uint64_t interval) {
+	nstime_t wakeup;
+#ifdef JEMALLOC_HAVE_PTHREAD_COND_TIMEDWAIT_MONOTONIC
+	struct timespec now;
+	clock_gettime(CLOCK_MONOTONIC, &now);
+	nstime_init2(&wakeup, now.tv_sec, now.tv_nsec);
+#else
+	struct timeval tv;
+	gettimeofday(&tv, NULL);
+	nstime_init2(&wakeup, tv.tv_sec, tv.tv_usec * 1000);
+#endif
+	nstime_iadd(&wakeup, interval);
+	ts->tv_sec = (size_t)nstime_sec(&wakeup);
+	ts->tv_nsec = (size_t)nstime_nsec(&wakeup);
+}
+
 static void
 background_thread_sleep(
     tsdn_t *tsdn, background_thread_info_t *info, uint64_t interval) {
@@ -199,11 +243,8 @@ 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) {
@@ -223,21 +264,16 @@ background_thread_sleep(
 		background_thread_wakeup_time_set(
 		    tsdn, info, nstime_ns(&next_wakeup));
 
-		nstime_t ts_wakeup;
-		nstime_copy(&ts_wakeup, &before_sleep);
-		nstime_iadd(&ts_wakeup, interval);
 		struct timespec ts;
-		ts.tv_sec = (size_t)nstime_sec(&ts_wakeup);
-		ts.tv_nsec = (size_t)nstime_nsec(&ts_wakeup);
+		background_thread_wakeup_ts_init(&ts, interval);
 
 		assert(!background_thread_indefinite_sleep(info));
 		ret = background_thread_cond_wait(info, &ts);
 		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 +787,7 @@ 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;
-		int ret = pthread_cond_init(&info->cond, NULL);
+		int ret = background_thread_cond_init(&info->cond);
 		assert(ret == 0);
 		background_thread_info_init(tsdn, info);
 		malloc_mutex_unlock(tsdn, &info->mtx);
@@ -869,7 +905,7 @@ background_thread_boot1(tsdn_t *tsdn, base_t *base) {
 		        malloc_mutex_address_ordered)) {
 			return true;
 		}
-		if (pthread_cond_init(&info->cond, NULL)) {
+		if (background_thread_cond_init(&info->cond)) {
 			return true;
 		}
 		malloc_mutex_lock(tsdn, &info->mtx);