Re: sched_clock() uses are broken

[George Anzinger <george@wildturkeyranch.net>]

Nicolas Pitre wrote:
> On Tue, 2 May 2006, Russell King wrote:
>
>
> > 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 large number which no longer represents the amount of time.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.
>
>
> Better yet the sched_clock() implementation just needs to return a value 
> shifted left so the wrap around always happens on 64 bits and the 
> difference between two consecutive samples is always right.
>
>
> > > > 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.
>
>
> ... on x86 that is.
>
>
> > > 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.
>
>
> Yet that counter isn't necessarily nanosecond based.  So rescaling the 
> returned value to nanosecs requires expensive divisions which could be 
> done only once within sched_clock_diff() instead of twice as often in 
> each sched_clock() calls.

Oh phooey!!  Scaling can be done with a mpy and a shift.  See the new clock 
code where the TSC (or what ever) is scaled to ns.


--
George Anzinger   george@wildturkeyranch.net
HRT (High-res-timers):  http://sourceforge.net/projects/high-res-timers/
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