[PATCH] fix the issue that the tick_nohz_get_sleep_length() function could return a negative value

["Zhou Ti (x2019cwm)" <x2019cwm@stfx.ca>]

Fix the issue that the tick_nohz_get_sleep_length() function could return a 
negative value.

The variable "dev->next_event" has a small possibility to be smaller than 
the variable "now" during running, which would result in a negative value 
of "*delta_next". The variable "next_event" also has a small posibility to 
be smaller than the variable "now". Both case could lead to a negative 
return of function tick_nohz_get_sleep_length().

Signed-off-by: Ti Zhou <x2019cwm@stfx.ca>
---
--- tip/kernel/time/tick-sched.c.orig	2021-01-20 05:34:25.151325912 -0400
+++ tip/kernel/time/tick-sched.c	2021-01-20 19:44:28.238538380 -0400
@@ -1142,6 +1142,9 @@ ktime_t tick_nohz_get_sleep_length(ktime
 
 	*delta_next = ktime_sub(dev->next_event, now);
 
+	if (unlikely(*delta_next < 0))
+		*delta_next = 0;
+
 	if (!can_stop_idle_tick(cpu, ts))
 		return *delta_next;
 
@@ -1156,6 +1159,9 @@ ktime_t tick_nohz_get_sleep_length(ktime
 	next_event = min_t(u64, next_event,
 			   hrtimer_next_event_without(&ts->sched_timer));
 
+	if (unlikely(next_event < now))
+		next_event = now;
+
 	return ktime_sub(next_event, now);
 }