Re: sched_clock() uses are broken

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

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