Re: sched_clock() uses are broken

Russell King
2006-05-02T16:50:19Z

On Tue, May 02, 2006 at 06:43:45PM +0200, Andi Kleen wrote:
> Russell King <rmk+lkml@arm.linux.org.uk> writes:
> >
> > However, this is not the case. On x86 with TSC, it returns a 54 bit
> > number. This means that when t1 < t0, time_passed_ns becomes a very
> > large number which no longer represents the amount of time.
>
> Good point. For a 1Ghz system this would happen every ~0.57 years.
>
> The problem is there is AFAIK no non destructive[1] way to find out how
> many bits the TSC has
>
> Destructive would be to overwrite it with -1 and see how many stick.
>
> > All uses in kernel/sched.c seem to be aflicted by this problem.
> >
> > There are several solutions to this - the most obvious being that we
> > need a function which returns the nanosecond difference between two
> > sched_clock() return values, and this function needs to know how to
> > handle the case where sched_clock() has wrapped.
>
> Ok it can be done with a simple test.
>
> >
> > IOW:
> >
> > t0 = sched_clock();
> > /* do something */
> > t1 = sched_clock();
> >
> > time_passed = sched_clock_diff(t1, t0);
> >
> > Comments?
>
> Agreed it's a problem, but probably a small one. At worst you'll get
> a small scheduling hickup every half year, which should be hardly
> that big an issue.
>
> Might chose to just ignore it with a big fat comment?

You're right assuming you have a 64-bit TSC, but ARM has at best a
32-bit cycle counter which rolls over about every 179 seconds - with
gives a range of values from sched_clock from 0 to 178956970625 or
0x29AAAAAA81.

That's rather more of a problem than having it happen every 208 days.

--
Russell King
Linux kernel 2.6 ARM Linux - http://www.arm.linux.org.uk/
maintainer of: 2.6 Serial core