4.9-stable review patch.  If anyone has any objections, please let me know.

------------------

From: David Engraf <redacted>


[ Upstream commit 1b8955bc5ac575009835e371ae55e7f3af2197a9 ]

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 <redacted>
Signed-off-by: John Stultz <redacted>
Signed-off-by: Sasha Levin <redacted>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 kernel/time/sched_clock.c |    5 +++++
 1 file changed, 5 insertions(+)

--- a/kernel/time/sched_clock.c
+++ b/kernel/time/sched_clock.c

@@ -205,6 +205,11 @@ sched_clock_register(u64 (*read)(void),
 
 	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;