2 files changed, 38 insertions, 6 deletions

diff --git a/kernel/itimer.c b/kernel/itimer.c
index 852c88ddd1f0..21adff7b2a17 100644
--- a/kernel/itimer.c
+++ b/kernel/itimer.c
@@ -42,7 +42,7 @@ static struct timeval itimer_get_remtime(struct hrtimer *timer)
 }
 
 static void get_cpu_itimer(struct task_struct *tsk, unsigned int clock_id,
-			   struct itimerval *value)
+			   struct itimerval *const value)
 {
 	cputime_t cval, cinterval;
 	struct cpu_itimer *it = &tsk->signal->it[clock_id];
@@ -127,14 +127,32 @@ enum hrtimer_restart it_real_fn(struct hrtimer *timer)
 	return HRTIMER_NORESTART;
 }
 
+static inline u32 cputime_sub_ns(cputime_t ct, s64 real_ns)
+{
+	struct timespec ts;
+	s64 cpu_ns;
+
+	cputime_to_timespec(ct, &ts);
+	cpu_ns = timespec_to_ns(&ts);
+
+	return (cpu_ns <= real_ns) ? 0 : cpu_ns - real_ns;
+}
+
 static void set_cpu_itimer(struct task_struct *tsk, unsigned int clock_id,
-			   struct itimerval *value, struct itimerval *ovalue)
+			   const struct itimerval *const value,
+			   struct itimerval *const ovalue)
 {
-	cputime_t cval, cinterval, nval, ninterval;
+	cputime_t cval, nval, cinterval, ninterval;
+	s64 ns_ninterval, ns_nval;
 	struct cpu_itimer *it = &tsk->signal->it[clock_id];
 
 	nval = timeval_to_cputime(&value->it_value);
+	ns_nval = timeval_to_ns(&value->it_value);
 	ninterval = timeval_to_cputime(&value->it_interval);
+	ns_ninterval = timeval_to_ns(&value->it_interval);
+
+	it->incr_error = cputime_sub_ns(ninterval, ns_ninterval);
+	it->error = cputime_sub_ns(nval, ns_nval);
 
 	spin_lock_irq(&tsk->sighand->siglock);
 
diff --git a/kernel/posix-cpu-timers.c b/kernel/posix-cpu-timers.c
index 9b2d5e4dc8c4..b60d644ea4b3 100644
--- a/kernel/posix-cpu-timers.c
+++ b/kernel/posix-cpu-timers.c
@@ -1070,6 +1070,8 @@ static void stop_process_timers(struct task_struct *tsk)
 	spin_unlock_irqrestore(&cputimer->lock, flags);
 }
 
+static u32 onecputick;
+
 static void check_cpu_itimer(struct task_struct *tsk, struct cpu_itimer *it,
 			     cputime_t *expires, cputime_t cur_time, int signo)
 {
@@ -1077,9 +1079,16 @@ static void check_cpu_itimer(struct task_struct *tsk, struct cpu_itimer *it,
 		return;
 
 	if (cputime_ge(cur_time, it->expires)) {
-		it->expires = it->incr;
-		if (!cputime_eq(it->expires, cputime_zero))
-			it->expires = cputime_add(it->expires, cur_time);
+		if (!cputime_eq(it->incr, cputime_zero)) {
+			it->expires = cputime_add(it->expires, it->incr);
+			it->error += it->incr_error;
+			if (it->error >= onecputick) {
+				it->expires = cputime_sub(it->expires,
+							jiffies_to_cputime(1));
+				it->error -= onecputick;
+			}
+		} else
+			it->expires = cputime_zero;
 
 		__group_send_sig_info(signo, SEND_SIG_PRIV, tsk);
 	}
@@ -1696,10 +1705,15 @@ static __init int init_posix_cpu_timers(void)
 		.nsleep = thread_cpu_nsleep,
 		.nsleep_restart = thread_cpu_nsleep_restart,
 	};
+	struct timespec ts;
 
 	register_posix_clock(CLOCK_PROCESS_CPUTIME_ID, &process);
 	register_posix_clock(CLOCK_THREAD_CPUTIME_ID, &thread);
 
+	cputime_to_timespec(jiffies_to_cputime(1), &ts);
+	onecputick = ts.tv_nsec;
+	WARN_ON(ts.tv_sec != 0);
+
 	return 0;
 }
 __initcall(init_posix_cpu_timers);