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

dev-aaront-org · Peter Zijlstra · commit f31dcb152a3d · 2023-04-21T13:24:21.000+02:00
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: 776f229 ("sched/clock: Make local_clock() noinstr") Signed-off-by: Aaron Thompson <dev@aaront.org> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Link: https://lkml.kernel.org/r/20230413175012.2201-1-dev@aaront.org
diff --git 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();
