linux-kernel - Re: [PATCH] clocksource: document some basic concepts

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Date:	Mon, 15 Nov 2010 15:06:29 -0500 (EST)
From:	Nicolas Pitre <nico@...xnic.net>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Linus Walleij <linus.walleij@...ricsson.com>,
	lkml <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Colin Cross <ccross@...gle.com>,
	John Stultz <johnstul@...ibm.com>,
	Ingo Molnar <mingo@...hat.com>,
	Rabin Vincent <rabin.vincent@...ricsson.com>
Subject: Re: [PATCH] clocksource: document some basic concepts

On Mon, 15 Nov 2010, Peter Zijlstra wrote:

> On Mon, 2010-11-15 at 11:33 +0100, Linus Walleij wrote:
> > +The sched_clock() function may wrap only on unsigned long long boundaries,
> > +i.e. after 64 bits. Since this is a nanosecond value this will mean it wraps
> > +after circa 585 years. (For most practical systems this means "never".)

This is not necessarily the case.  Some implementations require a 
scaling factor too, making the number of remaining bits smaller than 64.  
See arch/arm/mach-pxa/time.c:sched_clock() for example, which has a 
maximum range of 208 days.  Of course, in practice we don't really care 
if sched_clock() wraps each 208 days, unlike for clock-source.

> Currently true, John Stultz was going to look into ammending this by
> teaching the kernel/sched_clock.c bits about early wraps (and a way for
> architectures to specify this)
> 
> #define SCHED_CLOCK_WRAP_BITS 48
> 
> ...
> 
> #ifdef SCHED_CLOCK_WRAP_BITS
>   /* handle short wraps */
> #endif

Is this worth supporting?  I'd simply use the low 32 bits and extend it 
to 63 bits using cnt32_to_63(). If the low 32 bits are wrapping too 
fast, then just shifting them down a few positions first should do the 
trick.  That certainly would have a much faster result.

Nicolas
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