Hi Peter,
I have revisted this patch after modifying x86 sched_clock() to contigously
output tsc once it is setup early in boot, based on the latest suggestions
from Thomas.
On 18-06-25 10:55:43, Peter Zijlstra wrote:
> 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.
>
> > + 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);
Unfortunatly the above suggestion won't work. And here is why.
We have a call sequence like this:
start_kernel
sched_init()
sched_clock_init()
In this call sched_clock_running is set to 1. Which means
that sched_clock_cpu() starts doing the following sequence:
scd = cpu_sdc(cpu);
clock = sched_clock_local(scd);
Where we try to filter the output of sched_clock() based
on the value of scd. But, that won't work, because to get
this functionality, we need to have: timer initialized
that wakes up and updates scd, and we need timekeeping
initialized, so we can call ktime_get_ns(). Both of which
are called later.
...
timekeeping_init() After this we can call ktime_get_ns()
time_init() Here we configure x86_late_time_init pointer.
...
late_time_init()
x86_late_time_init()
x86_init.timers.timer_init()
hpet_time_init() Only after this call we finally start
getting clock interrupts, and can get precise output from
sched_clock_local().
The way I solved the above, is I changed sched_clock() to keep outputing
time based on early boot sched_clock() until sched_clock_init_late(), at
whic point everything is configured and we can switch to the permanent
clock, eventhough this happens after smp init.
If you have a better solution, please let me know.
Thank you,
Pavel
