timers, sched_clock: Update timeout for clock wrap

The scheduler clock framework may not use the correct timeout for the clock
wrap. This happens when a new clock driver calls sched_clock_register()
after the kernel called sched_clock_postinit(). In this case the clock wrap
timeout is too long thus sched_clock_poll() is called too late and the clock
already wrapped.

On my ARM system the scheduler was no longer scheduling any other task than
the idle task because the sched_clock() wrapped.
Signed-off-by: David Engraf <david.engraf@sysgo.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>

kernel/time/sched_clock.c

@@ -206,6 +206,11 @@ sched_clock_register(u64 (*read)(void), int bits, unsigned long rate)
 
 	update_clock_read_data(&rd);
 
+	if (sched_clock_timer.function != NULL) {
+		/* update timeout for clock wrap */
+		hrtimer_start(&sched_clock_timer, cd.wrap_kt, HRTIMER_MODE_REL);
+	}
+
 	r = rate;
 	if (r >= 4000000) {
 		r /= 1000000;