[PATCH 1/1] dynticks/preempt_rt: Fix a nohz_full entry failure in preempt_rt

[ramesh.thomas@intel.com]

From: Ramesh Thomas <ramesh.thomas@intel.com>

After all conditions to enter nohz_full state are met, if a timer event
is detected in the next period, nohz_full entry is aborted in
tick_nohz_next_event(). In the failure condition, this scenario keeps
repeating. This has been reproduced in preempt_rt kernel, however it may
not be limited to it.

Not bailing out if CONFIG_NO_HZ_FULL is defined, fixes the issue. Since
in NO_HZ_FULL mode, idle state is not entered at tick stoppage, the
pending timer event can be handled where the next timer is programmed in
tick_nohz_stop_tick(). This also simplifies the logic by keeping a
common path to handle the entry/exit of nohz_full state, whether the
tick was already stopped or not.

(The entire check for the presence of a timer event in the next period
can possibly be skipped for NO_HZ_FULL)

Signed-off-by: Ramesh Thomas <ramesh.thomas@intel.com>
---
 kernel/time/tick-sched.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 308a98c28ee8..3798dbc5f082 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -739,10 +739,12 @@ static ktime_t tick_nohz_next_event(struct tick_sched *ts, int cpu)
 		 * We've not stopped the tick yet, and there's a timer in the
 		 * next period, so no point in stopping it either, bail.
 		 */
+#ifndef CONFIG_NO_HZ_FULL
 		if (!ts->tick_stopped) {
 			ts->timer_expires = 0;
-			goto out;
+			return 0;
 		}
+#endif
 	}
 
 	/*
@@ -763,7 +765,6 @@ static ktime_t tick_nohz_next_event(struct tick_sched *ts, int cpu)
 
 	ts->timer_expires = min_t(u64, expires, next_tick);
 
-out:
 	return ts->timer_expires;
 }
 
-- 
2.26.2