Re: Scalability enhancements for gettimeofday

[David Mosberger <davidm@napali.hpl.hp.com>]

>>>>> On Tue, 8 Jun 2004 14:48:20 -0700 (PDT), Christoph Lameter <christoph@lameter.com> said:

  Christoph> Please show me how last_itc can *receive* a bogus
  Christoph> value. What you are saying cannot happen because last_itc
  Christoph> does not depend in any way on variables modified by the
  Christoph> timer interrupt. Last_itc is updated using a cmpxchg
  Christoph> which will deal with the problem of multiple CPUs
  Christoph> updating the value.

Ah, I see now what you mean.  Yes, tracking the maximum ITC rather
than a relative offsets is a nice trick that solves the problem of the
ITC-based getoffset not being "restartable".

What threw me off is that you continue to read jiffies and
wall_jiffies in the do/while-loop for the cmpxchg.  I don't see why
that's necessary/helpful.  If you get a tick, you'll throw away the
result of the routine anyhow so you might just as well move it out of
the loop.

So I think your patch does work.  I'd only ask that you replace the
explicit "check-for-earlier while allowing for wraparound" with a call
to the time_before() macro (or at least the equivalent open-code, if
you're nervous about using jiffies-related macros for non-jiffy
values).

Thanks,

	--david