From: Thomas Gleixner <tglx@linutronix.de>
Date: Tue, 21 Oct 2008 18:17:35 +0000 (+0200)
Subject: NOHZ: fix thinko in the timer restart code path
X-Git-Tag: v2.6.28-rc1~38^2
X-Git-Url: http://pilppa.com/gitweb/?a=commitdiff_plain;h=c4bd822e7b12a9008241d76db45b665f2fef180c;p=linux-2.6-omap-h63xx.git

NOHZ: fix thinko in the timer restart code path

commit fb02fbc14d17837b4b7b02dbb36142c16a7bf208 (NOHZ: restart tick
device from irq_enter())

solves the problem of stale jiffies when long running softirqs happen
in a long idle sleep period, but it has a major thinko in it:

When the interrupt which came in _is_ the timer interrupt which should
expire ts->sched_timer then we cancel and rearm the timer _before_ it
gets expired in hrtimer_interrupt() to the next period. That means the
call back function is not called. This game can go on for ever :(

Prevent this by making sure to only rearm the timer when the expiry
time is more than one tick_period away. Otherwise keep it running as
it is either already expired or will expiry at the right point to
update jiffies.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Venkatesch Pallipadi <venkatesh.pallipadi@intel.com>
---

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 0581c11fe6c..727c1ae0517 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -567,11 +567,21 @@ static void tick_nohz_switch_to_nohz(void)
 static void tick_nohz_kick_tick(int cpu)
 {
 	struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);
+	ktime_t delta, now;
 
 	if (!ts->tick_stopped)
 		return;
 
-	tick_nohz_restart(ts, ktime_get());
+	/*
+	 * Do not touch the tick device, when the next expiry is either
+	 * already reached or less/equal than the tick period.
+	 */
+	now = ktime_get();
+	delta =	ktime_sub(ts->sched_timer.expires, now);
+	if (delta.tv64 <= tick_period.tv64)
+		return;
+
+	tick_nohz_restart(ts, now);
 }
 
 #else