sched/clock: Close a hole in sched_clock_init()

All data required for the 'unstable' sched_clock must be set-up _before_
enabling it -- setting sched_clock_running. This includes the
__gtod_offset but also a recent scd stamp.

Make the gtod-offset update also set the csd stamp -- it requires the
same two clock reads _anyway_. This doesn't hurt in the
sched_clock_tick_stable() case and ensures sched_clock_init() gets
everything set-up before use.

Also switch to unconditional IRQ-disable/enable because the static key
stuff already requires this is not ran with IRQs disabled.

Fixes: 857baa87b642 ("sched/clock: Enable sched clock early")
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Pavel Tatashin <pasha.tatashin@oracle.com>
Cc: steven.sistare@oracle.com
Cc: daniel.m.jordan@oracle.com
Cc: linux@armlinux.org.uk
Cc: schwidefsky@de.ibm.com
Cc: heiko.carstens@de.ibm.com
Cc: john.stultz@linaro.org
Cc: sboyd@codeaurora.org
Cc: hpa@zytor.com
Cc: douly.fnst@cn.fujitsu.com
Cc: prarit@redhat.com
Cc: feng.tang@intel.com
Cc: pmladek@suse.com
Cc: gnomes@lxorguk.ukuu.org.uk
Cc: linux-s390@vger.kernel.org
Cc: boris.ostrovsky@oracle.com
Cc: jgross@suse.com
Cc: pbonzini@redhat.com
Link: https://lkml.kernel.org/r/20180720080911.GM2494@hirez.programming.kicks-ass.net

kernel/sched/clock.c

@@ -197,13 +197,14 @@ void clear_sched_clock_stable(void)
 
 static void __sched_clock_gtod_offset(void)
 {
-	__gtod_offset = (sched_clock() + __sched_clock_offset) - ktime_get_ns();
+	struct sched_clock_data *scd = this_scd();
+
+	__scd_stamp(scd);
+	__gtod_offset = (scd->tick_raw + __sched_clock_offset) - scd->tick_gtod;
 }
 
 void __init sched_clock_init(void)
 {
-	unsigned long flags;
-
 	/*
 	 * Set __gtod_offset such that once we mark sched_clock_running,
 	 * sched_clock_tick() continues where sched_clock() left off.
@@ -211,16 +212,11 @@ void __init sched_clock_init(void)
 	 * Even if TSC is buggered, we're still UP at this point so it
 	 * can't really be out of sync.
 	 */
-	local_irq_save(flags);
+	local_irq_disable();
 	__sched_clock_gtod_offset();
-	local_irq_restore(flags);
+	local_irq_enable();
 
 	static_branch_inc(&sched_clock_running);
-
-	/* Now that sched_clock_running is set adjust scd */
-	local_irq_save(flags);
-	sched_clock_tick();
-	local_irq_restore(flags);
 }
 /*
  * We run this as late_initcall() such that it runs after all built-in drivers,