Re: [PATCH v3 3/3] arm64: Early boot time stamps

[Pavel Tatashin <pasha.tatashin@soleen.com>]

> I still think this approach is flawed. You provide the kernel with a
> potentially broken sched_clock that may jump back and forth until the
> workaround kicks in. Nobody expects this.
>
> Instead, I'd suggest you allow for a something other than local_clock()
> to be used for the time stamping until a properly working sched_clock
> gets registered.
>
> This way, you'll only impact the timestamps when running on a broken system.

I think, given that on other platforms sched_clock() is already used
early, it is not a good idea to invent a different clock just for time
stamps.

We could limit arm64 approach only for chips where cntvct_el0 is
working: i.e. frequency is known, and the clock is stable, meaning
cannot go backward. Perhaps we would start early clock a little later,
but at least it will be available for the sane chips. The only
question, where during boot time this is known.

Another approach is to modify sched_clock() in
kernel/time/sched_clock.c to never return backward value during boot.

1. Rename  current implementation of sched_clock() to sched_clock_raw()
2. New sched_clock() would look like this:

u64 sched_clock(void)
{
   if (static_branch(early_unstable_clock))
      return sched_clock_unstable();
   else
      return sched_clock_raw();
}

3. sched_clock_unstable() would look like this:

u64 sched_clock_unstable(void)
{
again:
  static u64 old_clock;
  u64 new_clock = sched_clock_raw();
  static u64 old_clock_read =   READ_ONCE(old_clock);
  /* It is ok if time does not progress, but don't allow to go backward */
  if (new_clock < old_clock_read)
    return old_clock_read;
   /* update the old_clock value */
   if (cmpxchg64(&old_clock, old_clock_read, new_clock) != old_clock_read)
      goto again;
   return new_clock;
}

Pasha