Re: sched_clock() uses are broken

[Russell King <rmk+lkml@arm.linux.org.uk>]

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 large number which no longer represents the amount of time.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