On Thu, Jun 21, 2018 at 05:25:17PM -0400, Pavel Tatashin wrote:
> 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.
There are !x86 architectures that use this code and might not expect to
have their sched_clock() called quite that early. Please verify.
> Signed-off-by: Pavel Tatashin <pasha.tatas...@oracle.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 */
This is wrong... or at least it smells horribly.
This is not the point where we transition from early to unstable, that
is in fact in sched_clock_init.
This function, sched_clock_init_late(), is where we attempt to
transition from unstable to stable. And this is _waaaay_ after SMP init.
> + local_irq_disable();
> + __gtod_offset = sched_clock() + __sched_clock_offset - ktime_get_ns();
> + local_irq_enable();
This might work in sched_clock_init(), which is pre-SMP.
> 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;
And then this remains !sched_clock_running, except instead of 0, you
then return sched_clock() + __sched_clock_offset;
> preempt_disable_notrace();
> scd = cpu_sdc(cpu);
