Date: Tue, 02 May 2006 15:05:22 -0400 (EDT)
From: Nicolas Pitre <nico@cam.org>
Subject: Re: sched_clock() uses are broken
In-reply-to: <20060502185555.GB4223@flint.arm.linux.org.uk>
To: Russell King <rmk+lkml@arm.linux.org.uk>
Cc: Andi Kleen <ak@suse.de>, linux-kernel@vger.kernel.org
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On Tue, 2 May 2006, Russell King wrote:

> 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.

If we're discussing the addition of a sched_clock_diff(), why whouldn't 
shed_clock() return anything it wants in that context?  It could be 
redefined to have a return value meaningful only to shed_clock_diff()?


Nicolas

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