Re: sched_clock

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

>>>>> On Fri, 04 Jun 2004 13:11:26 +0200, Zoltan Menyhart <Zoltan.Menyhart_AT_bull.net@nospam.org> said:

  Zoltan> David Mosberger wrote:
  >>  Yes, you're right, there is an intermediate-result overflow
  >> problem in sched_clock() that I missed.  How does the attached
  >> patch work for you?

  Zoltan> Thank you, it is O.K.

Thanks for checking it out.

  Zoltan> On the other hand, we might have similar problems in
  Zoltan> fsys.S. Some comments like the one below makes me worry a
  Zoltan> bit:

  Zoltan> 	// if now < last_tick, set p7 = 1, p8 = 0

  Zoltan> I cannot really say that I fully understand what is in
  Zoltan> fsys.S, but should not "now" always be after no matter what
  Zoltan> time stamp, last tick ?

No, the "<" there is done with 64-bit modular arithmetic so this code
is fine (it's the equivalent of the time_before() macro).

The ia64-specific code is very careful to work correctly even when the
ITC wraps around.  sched_clock() is a (platform-independent) exception
and it is OK because, in the worst case, an overflow will lead to a
scheduling hiccup.  Basically, the scheduler will simply "think" that
a task slept for NS_MAX_SLEEP_AVG when it may have slept much less
(there may be some other minor hiccups, but that should be pretty much
it, AFAICT).

	--david