[tip:timers/urgent] tick: Make sure tick timer is active when bypassing reprogramming

[tip-bot for Frederic Weisbecker <tipbot@zytor.com>]

Commit-ID:  22aa2ad45fd8a6ef56eb60038fc0ac7059c0a986
Gitweb:     http://git.kernel.org/tip/22aa2ad45fd8a6ef56eb60038fc0ac7059c0a986
Author:     Frederic Weisbecker <fweisbec@gmail.com>
AuthorDate: Fri, 21 Apr 2017 16:00:55 +0200
Committer:  Thomas Gleixner <tglx@linutronix.de>
CommitDate: Sun, 23 Apr 2017 13:33:18 +0200

tick: Make sure tick timer is active when bypassing reprogramming

So far we have run into too much troubles with the optimization path
that skips reprogramming the clock on IRQ exit when the expiration
deadline hasn't changed. If by accident the cached deadline happens to
be out of sync with the hardware deadline, the buggy result and its
cause are hard to investigate. So lets detect and warn about the issue
early.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Acked-by: Rik van Riel <riel@redhat.com>
Cc: James Hartsock <hartsjc@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: stable@vger.kernel.org
Cc: Tim Wright <tim@binbash.co.uk>
Cc: Pavel Machek <pavel@ucw.cz>
Link: http://lkml.kernel.org/r/1492783255-5051-3-git-send-email-fweisbec@gmail.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

---
 kernel/time/tick-sched.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 502b320..be7ca4d 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -783,8 +783,13 @@ static ktime_t tick_nohz_stop_sched_tick(struct tick_sched *ts,
 	tick = expires;
 
 	/* Skip reprogram of event if its not changed */
-	if (ts->tick_stopped && (expires == ts->next_tick))
-		goto out;
+	if (ts->tick_stopped && (expires == ts->next_tick)) {
+		/* Sanity check: make sure clockevent is actually programmed */
+		if (likely(dev->next_event <= ts->next_tick))
+			goto out;
+
+		WARN_ON_ONCE(1);
+	}
 
 	/*
 	 * nohz_stop_sched_tick can be called several times before