[tip: sched/core] sched/clock: Fix local_clock() before sched_clock_init()

[tip-bot2 for Aaron Thompson]

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     f31dcb152a3d0816e2f1deab4e64572336da197d
Gitweb:        https://git.kernel.org/tip/f31dcb152a3d0816e2f1deab4e64572336da197d
Author:        Aaron Thompson <dev@xxxxxxxxxx>
AuthorDate:    Thu, 13 Apr 2023 17:50:12 
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Fri, 21 Apr 2023 13:24:21 +02:00

sched/clock: Fix local_clock() before sched_clock_init()

Have local_clock() return sched_clock() if sched_clock_init() has not
yet run. sched_clock_cpu() has this check but it was not included in the
new noinstr implementation of local_clock().

The effect can be seen on x86 with CONFIG_PRINTK_TIME enabled, for
instance. scd->clock quickly reaches the value of TICK_NSEC and that
value is returned until sched_clock_init() runs.

dmesg without this patch:

    [    0.000000] kvm-clock: ...
    [    0.000002] kvm-clock: ...
    [    0.000672] clocksource: ...
    [    0.001000] tsc: ...
    [    0.001000] e820: ...
    [    0.001000] e820: ...
     ...
    [    0.001000] ..TIMER: ...
    [    0.001000] clocksource: ...
    [    0.378956] Calibrating delay loop ...
    [    0.379955] pid_max: ...

dmesg with this patch:

    [    0.000000] kvm-clock: ...
    [    0.000001] kvm-clock: ...
    [    0.000675] clocksource: ...
    [    0.002685] tsc: ...
    [    0.003331] e820: ...
    [    0.004190] e820: ...
     ...
    [    0.421939] ..TIMER: ...
    [    0.422842] clocksource: ...
    [    0.424582] Calibrating delay loop ...
    [    0.425580] pid_max: ...

Fixes: 776f22913b8e ("sched/clock: Make local_clock() noinstr")
Signed-off-by: Aaron Thompson <dev@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/20230413175012.2201-1-dev@xxxxxxxxxx
---
 kernel/sched/clock.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/sched/clock.c b/kernel/sched/clock.c
index 5732fa7..b5cc2b5 100644
--- a/kernel/sched/clock.c
+++ b/kernel/sched/clock.c
@@ -300,6 +300,9 @@ noinstr u64 local_clock(void)
 	if (static_branch_likely(&__sched_clock_stable))
 		return sched_clock() + __sched_clock_offset;
 
+	if (!static_branch_likely(&sched_clock_running))
+		return sched_clock();
+
 	preempt_disable_notrace();
 	clock = sched_clock_local(this_scd());
 	preempt_enable_notrace();