kernel/hung_task.c: use timeout diff when timeout is updated

When new timeout is written to /proc/sys/kernel/hung_task_timeout_secs,
khungtaskd is interrupted and again sleeps for full timeout duration.

This means that hang task will not be checked if new timeout is written
periodically within old timeout duration and/or checking of hang task
will be delayed for up to previous timeout duration.  Fix this by
remembering last time khungtaskd checked hang task.

This change will allow other watchdog tasks (if any) to share khungtaskd
by sleeping for minimal timeout diff of all watchdog tasks.  Doing more
watchdog tasks from khungtaskd will reduce the possibility of printk()
collisions by multiple watchdog threads.
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Aaron Tomlin <atomlin@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/hung_task.c

@@ -185,10 +185,12 @@ static void check_hung_uninterruptible_tasks(unsigned long timeout)
 	rcu_read_unlock();
 }
 
-static unsigned long timeout_jiffies(unsigned long timeout)
+static long hung_timeout_jiffies(unsigned long last_checked,
+				 unsigned long timeout)
 {
 	/* timeout of 0 will disable the watchdog */
-	return timeout ? timeout * HZ : MAX_SCHEDULE_TIMEOUT;
+	return timeout ? last_checked - jiffies + timeout * HZ :
+		MAX_SCHEDULE_TIMEOUT;
 }
 
 /*
@@ -224,18 +226,21 @@ EXPORT_SYMBOL_GPL(reset_hung_task_detector);
  */
 static int watchdog(void *dummy)
 {
+	unsigned long hung_last_checked = jiffies;
+
 	set_user_nice(current, 0);
 
 	for ( ; ; ) {
 		unsigned long timeout = sysctl_hung_task_timeout_secs;
+		long t = hung_timeout_jiffies(hung_last_checked, timeout);
 
-		while (schedule_timeout_interruptible(timeout_jiffies(timeout)))
-			timeout = sysctl_hung_task_timeout_secs;
-
-		if (atomic_xchg(&reset_hung_task, 0))
-			continue;
-
+		if (t <= 0) {
+			if (!atomic_xchg(&reset_hung_task, 0))
 				check_hung_uninterruptible_tasks(timeout);
+			hung_last_checked = jiffies;
+			continue;
+		}
+		schedule_timeout_interruptible(t);
 	}
 
 	return 0;