sched/clock: Enable sched clock early

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

Signed-off-by: Pavel Tatashin <pasha.tatashin@oracle.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
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: peterz@infradead.org
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/20180719205545.16512-24-pasha.tatashin@oracle.com

2 files changed, 20 insertions, 2 deletions

diff --git a/init/main.c b/init/main.c
index 162d931c9511..ff0a24170b95 100644
--- a/init/main.c
+++ b/init/main.c
@@ -642,7 +642,6 @@ asmlinkage __visible void __init start_kernel(void)
         softirq_init();
         timekeeping_init();
         time_init();
-        sched_clock_init();
         printk_safe_init();
         perf_event_init();
         profile_init();
@@ -697,6 +696,7 @@ asmlinkage __visible void __init start_kernel(void)
         acpi_early_init();
         if (late_time_init)
                 late_time_init();
+        sched_clock_init();
         calibrate_delay();
         pid_idr_init();
         anon_vma_init();
diff --git a/kernel/sched/clock.c b/kernel/sched/clock.c
index 0e9dbb2d9aea..422cd63f8f17 100644
--- a/kernel/sched/clock.c
+++ b/kernel/sched/clock.c
@@ -202,7 +202,25 @@ static void __sched_clock_gtod_offset(void)
 
 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.
+         *
+         * 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);
+        __sched_clock_gtod_offset();
+        local_irq_restore(flags);
+
         sched_clock_running = 1;
+
+        /* 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,
@@ -356,7 +374,7 @@ u64 sched_clock_cpu(int cpu)
                 return sched_clock() + __sched_clock_offset;
 
         if (unlikely(!sched_clock_running))
-                return 0ull;
+                return sched_clock();
 
         preempt_disable_notrace();
         scd = cpu_sdc(cpu);