timers: Consider slack value in mod_timer()

There is an optimization which does not update the timer if the timer
was pending and the expiration time was unchanged.

Since commit 3bbb9ec9 ("timers: Introduce the concept of timer slack
for legacy timers") this optimization is no longer applied for timers
where the expiration time got extended due to the slack value. So we
need to check again after the expiration time might have been updated.

[ tglx: Made it a single check by applying slack first and sorting
out the slack = 0 value (all timeouts < 256 jiffies) early ]

Signed-off-by: Sebastian Andrzej Siewior <[e-mail]>
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Thomas Gleixner <[e-mail]>

kernel/timer.c

···
 	unsigned long expires_limit, mask;
 	int bit;
 
-	expires_limit = expires;
-
 	if (timer->slack >= 0) {
 		expires_limit = expires + timer->slack;
 	} else {
-		unsigned long now = jiffies;
+		long delta = expires - jiffies;
 
-		/* No slack, if already expired else auto slack 0.4% */
-		if (time_after(expires, now))
-			expires_limit = expires + (expires - now)/256;
+		if (delta < 256)
+			return expires;
+
+		expires_limit = expires + delta / 256;
 	}
 	mask = expires ^ expires_limit;
 	if (mask == 0)
···
  */
 int mod_timer(struct timer_list *timer, unsigned long expires)
 {
+	expires = apply_slack(timer, expires);
+
 	/*
 	 * This is a common optimization triggered by the
 	 * networking code - if the timer is re-modified
···
 	 */
 	if (timer_pending(timer) && timer->expires == expires)
 		return 1;
-
-	expires = apply_slack(timer, expires);
 
 	return __mod_timer(timer, expires, false, TIMER_NOT_PINNED);
 }