Subject: Re: [PATCH] clocksource: document some basic concepts
From: john stultz <johnstul@us.ibm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Linus Walleij <linus.walleij@stericsson.com>, linux-kernel@vger.kernel.org,
        Thomas Gleixner <tglx@linutronix.de>, Nicolas Pitre <nico@fluxnic.net>,
        Colin Cross <ccross@google.com>, Ingo Molnar <mingo@redhat.com>,
        Rabin Vincent <rabin.vincent@stericsson.com>
In-Reply-To: <1289818094.2109.487.camel@laptop>
References: <1289817228-14838-1-git-send-email-linus.walleij@stericsson.com>
	 <1289818094.2109.487.camel@laptop>
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Date: Mon, 15 Nov 2010 11:48:37 -0800
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On Mon, 2010-11-15 at 11:48 +0100, Peter Zijlstra wrote:
> On Mon, 2010-11-15 at 11:33 +0100, Linus Walleij wrote:
> > +sched_clock()
> > +-------------
> > +
> > +In addition to the clock sources and clock events there is a special weak
> > +function in the kernel called sched_clock(). This function shall return the
> > +number of nanoseconds since the system was started. An architecture may or
> > +may not provide an implementation of sched_clock() on its own.
> > +
> > +As the name suggests, sched_clock() is used for scheduling the system,
> > +determining the absolute timeslice for a certain process in the CFS scheduler
> > +for example. It is also used for printk timestamps when you have selected to
> > +include time information in printk for things like bootcharts.
> > +
> > +Compared to clock sources, sched_clock() has to be very fast: it is called
> > +much more often, especially by the scheduler. If you have to do trade-offs
> > +between accuracy compared to the clock source, you may sacrifice accuracy
> > +for speed in sched_clock(). It however require the same basic characteristics
> > +as the clock source, i.e. it has to be monotonic.
> 
> Not so, we prefer it be synchronized and monotonic, but we don't require
> so, see below.
> 
> > +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".)
> 
> 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)

I'd like to, although at the moment I don't have much space on my plate
to do this, so in the mean time, if someone has time and interest into
looking at this, ping me and I can lay out the basics of what likely
should be done.

thanks
-john


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