linux-kernel - [PATCH 6/7] Cpuidle: Deal with timer expiring in the past

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Date:	Mon, 24 Feb 2014 08:29:36 +0200
From:	Tuukka Tikkanen <tuukka.tikkanen@...aro.org>
To:	linux-pm@...r.kernel.org, rjw@...ysocki.net
Cc:	daniel.lezcano@...aro.org, tuukka.tikkanen@...aro.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH 6/7] Cpuidle: Deal with timer expiring in the past

Sometimes (fairly often) when the cpuidle menu governor is making a decision
about idle state to enter the next timer for the cpu appears to expire in
the past. The menu governor expects the expiry to always be in the future
and in fact stores the time delta in an unsigned variable. However, when
the expiry is in the past, the value returned by tick_nohz_get_sleep_length
can be negative. This patch prevents using negative values, instead making
the governor return immediately similar to having latency requirement set
to 0.

Note: As with latency == 0, the return value is 0 with no check to see if
the state 0 has been disabled or not.

Signed-off-by: Tuukka Tikkanen <tuukka.tikkanen@...aro.org>
---
 drivers/cpuidle/governors/menu.c |   10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/drivers/cpuidle/governors/menu.c b/drivers/cpuidle/governors/menu.c
index 71b5232..c414468 100644
--- a/drivers/cpuidle/governors/menu.c
+++ b/drivers/cpuidle/governors/menu.c
@@ -302,8 +302,16 @@ static int menu_select(struct cpuidle_driver *drv, struct cpuidle_device *dev)
 	if (unlikely(latency_req == 0))
 		return 0;
 
-	/* determine the expected residency time, round up */
+	/*
+	 * Determine the expected residency time. If the time is negative,
+	 * a timer interrupt has probably just expired after disabling
+	 * interrupts. Return as quickly as possible in the most shallow
+	 * state possible. tv_nsec is always positive, so only check the
+	 * seconds.
+	 */
 	t = ktime_to_timespec(tick_nohz_get_sleep_length());
+	if (t.tv_sec < 0)
+		return 0;
 	data->next_timer_us =
 		t.tv_sec * USEC_PER_SEC + t.tv_nsec / NSEC_PER_USEC;
 
-- 
1.7.9.5

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