linux-kernel - [PATCH v12 10/11] sched: early boot clock

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Message-Id: <20180621212518.19914-11-pasha.tatashin@oracle.com>
Date:   Thu, 21 Jun 2018 17:25:17 -0400
From:   Pavel Tatashin <pasha.tatashin@...cle.com>
To:     steven.sistare@...cle.com, daniel.m.jordan@...cle.com,
        linux@...linux.org.uk, schwidefsky@...ibm.com,
        heiko.carstens@...ibm.com, john.stultz@...aro.org,
        sboyd@...eaurora.org, x86@...nel.org, linux-kernel@...r.kernel.org,
        mingo@...hat.com, tglx@...utronix.de, hpa@...or.com,
        douly.fnst@...fujitsu.com, peterz@...radead.org, prarit@...hat.com,
        feng.tang@...el.com, pmladek@...e.com, gnomes@...rguk.ukuu.org.uk,
        linux-s390@...r.kernel.org
Subject: [PATCH v12 10/11] sched: early boot clock

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@...cle.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 10c83e73837a..f034392b0f6c 100644
--- a/kernel/sched/clock.c
+++ b/kernel/sched/clock.c
@@ -205,6 +205,11 @@ 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.
@@ -350,8 +355,9 @@ 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);
-- 
2.17.1