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Message-ID: <17874.21860.598842.609322@cargo.ozlabs.ibm.com>
Date: Wed, 14 Feb 2007 11:18:44 +1100
From: Paul Mackerras <paulus@samba.org>
To: Andi Kleen <ak@suse.de>
Cc: Christoph Lameter <clameter@sgi.com>,
       Arjan van de Ven <arjan@infradead.org>,
       Vojtech Pavlik <vojtech@suse.cz>, Jiri Bohac <jbohac@suse.cz>,
       linux-kernel@vger.kernel.org, ssouhlal@freebsd.org, tglx@linutronix.de,
       johnstul@us.ibm.com, zippel@linux-m68k.org, andrea@suse.de
Subject: Re: [patch 4/9] Remove the TSC synchronization on SMP machines
In-Reply-To: <200702131820.15180.ak@suse.de>
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	<Pine.LNX.4.64.0702130906530.23634@schroedinger.engr.sgi.com>
	<200702131820.15180.ak@suse.de>
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Andi Kleen writes:

> Just to avoid spreading misinformation: modulo some new broken hardware
> (which we always try to work around when found) i386/x86-64 gettimeofday
> is monotonic.  AFAIK on the currently known hardware it should be generally
> ok.
> 
> However ntpd can always screw you up, but that's inherent in the design.

On powerpc we manage to keep gettimeofday monotonic even when ntpd is
adjusting the clock.  We have 3 parameters used to convert a value
from the timebase register to the time of day, and these parameters
are adjusted if necessary at the beginning of each tick, based on the
value returned by current_tick_length().  The point is that
current_tick_length() tells you at the *beginning* of each tick how
much time will be added on to xtime at the *end* of that tick, and
that makes it possible to aim the interpolation to hit the same value
as xtime at the end of each tick.

Clearly if you make a discrete jump backwards with settimeofday or
adjtime, it's impossible to keep gettimeofday monotonic, but apart
from that it's monotonic on powerpc.

At least, that's the way it's supposed to work.  I hope the recent
timekeeping changes haven't broken it. :)

Paul.
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