Re: sched_clock() uses are broken

Russell King
2006-05-02T18:56:04Z

On Tue, May 02, 2006 at 01:18:25PM -0400, Nicolas Pitre wrote:
> On Tue, 2 May 2006, Andi Kleen wrote:
>
> > On Tuesday 02 May 2006 18:50, Russell King wrote:
> >
> > > 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.
> >
> > Ok but you know it's always 32bit right? You can fix it up then
> > with your proposal of a sched_diff()
> >
> > The problem would be fixing it up with a unknown number of bits.
>
> Just shift it left so you know you always have the most significant bits
> valid. The sched_diff() would take care of scaling it back to nanosecs.

sched_clock is currently defined to return nanoseconds so this isn't
a possibility.

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