[PATCH] no_idle_hz (s390/xen) 2.6.16.13: fix next_timer_interrupt() when timer pending

[Emmanuel Ackaouy <ack@xensource.com>]

Fix next_timer_interrupt() to return the expired timeout of any
pending timer instead of the default "nothing scheduled" timeout
value of jiffies+MAX_JIFFY_OFFSET. See comment in patch for details.

Signed-off-by: Emmanuel Ackaouy <ack@xensource.com>


diff -pruN pristine-linux-2.6.16.13/kernel/timer.c linux-2.6.16.13-xen/kernel/timer.c
--- pristine-linux-2.6.16.13/kernel/timer.c	2006-05-02 22:38:44.000000000 +0100
+++ linux-2.6.16.13-xen/kernel/timer.c	2006-06-28 21:38:58.000000000 +0100
@@ -555,7 +555,17 @@ found:
 	}
 	spin_unlock(&base->t_base.lock);
 
-	if (time_before(hr_expires, expires))
+	/*
+	 * If timers are pending, "expires" will be in the recent past
+	 * of "jiffies". If there are no hr_timers registered, "hr_expires"
+	 * will be "jiffies + MAX_JIFFY_OFFSET"; this is *just* short of being
+	 * considered to be before "jiffies". This makes it very likely that
+	 * "hr_expires" *will* be considered to be before "expires".
+	 * So we must check when there are pending timers (expires <= jiffies)
+	 * to ensure that we don't accidently tell the caller that there is
+	 * nothing scheduled until half an epoch (MAX_JIFFY_OFFSET)!
+	 */
+	if (time_before(jiffies, expires) && time_before(hr_expires, expires))
 		return hr_expires;
 
 	return expires;
-
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