Patch "sched/clock: Fix local_clock() before sched_clock_init()" has been added to the 6.3-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

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

to the 6.3-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     sched-clock-fix-local_clock-before-sched_clock_init.patch
and it can be found in the queue-6.3 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit 0c482f0d2e133620ada2bcdc41efebc744a09e05
Author: Aaron Thompson <dev@xxxxxxxxxx>
Date:   Thu Apr 13 17:50:12 2023 +0000

    sched/clock: Fix local_clock() before sched_clock_init()
    
    [ Upstream commit f31dcb152a3d0816e2f1deab4e64572336da197d ]
    
    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
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/kernel/sched/clock.c b/kernel/sched/clock.c
index 5732fa75ebab2..b5cc2b53464de 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();