[PATCH v8 4/6] sched: early boot clock

[Pavel Tatashin <pasha.tatashin@oracle.com>]

Allow sched_clock() to be used before schec_clock_init() and
sched_clock_init_late() are called. This provides us with a way to get
early boot timestamps on machines with unstable clocks.

Signed-off-by: Pavel Tatashin <pasha.tatashin@oracle.com>
---
 kernel/sched/clock.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/clock.c b/kernel/sched/clock.c
index e086babe6c61..8bc603951c2d 100644
--- a/kernel/sched/clock.c
+++ b/kernel/sched/clock.c
@@ -217,6 +217,11 @@ void clear_sched_clock_stable(void)
  */
 static int __init sched_clock_init_late(void)
 {
+	/* Transition to unstable clock from early clock */
+	local_irq_disable();
+	__gtod_offset = sched_clock() + __sched_clock_offset - ktime_get_ns();
+	local_irq_enable();
+
 	sched_clock_running = 2;
 	/*
 	 * Ensure that it is impossible to not do a static_key update.
@@ -362,8 +367,9 @@ u64 sched_clock_cpu(int cpu)
 	if (sched_clock_stable())
 		return sched_clock() + __sched_clock_offset;
 
-	if (unlikely(!sched_clock_running))
-		return 0ull;
+	/* Use early clock until sched_clock_init_late() */
+	if (unlikely(sched_clock_running < 2))
+		return sched_clock() + __sched_clock_offset;
 
 	preempt_disable_notrace();
 	scd = cpu_sdc(cpu);
-- 
2.15.0