linux-kernel - [PATCH 2/2] tick: Make sure tick timer is active when bypassing reprogramming

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Message-Id: <1492783255-5051-3-git-send-email-fweisbec@gmail.com>
Date:   Fri, 21 Apr 2017 16:00:55 +0200
From:   Frederic Weisbecker <fweisbec@...il.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Rik van Riel <riel@...hat.com>,
        James Hartsock <hartsjc@...hat.com>, stable@...r.kernel.org,
        Tim Wright <tim@...bash.co.uk>, Pavel Machek <pavel@....cz>
Subject: [PATCH 2/2] 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.

Acked-by: Rik van Riel <riel@...hat.com>
Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
Cc: Tim Wright <tim@...bash.co.uk>
Cc: Pavel Machek <pavel@....cz>
Cc: James Hartsock <hartsjc@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...nel.org>
---
 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
-- 
2.7.4