linux-kernel - Re: [patch 4/9] Remove the TSC synchronization on SMP machines

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Date:	Wed, 14 Feb 2007 11:18:44 +1100
From:	Paul Mackerras <paulus@...ba.org>
To:	Andi Kleen <ak@...e.de>
Cc:	Christoph Lameter <clameter@....com>,
	Arjan van de Ven <arjan@...radead.org>,
	Vojtech Pavlik <vojtech@...e.cz>, Jiri Bohac <jbohac@...e.cz>,
	linux-kernel@...r.kernel.org, ssouhlal@...ebsd.org,
	tglx@...utronix.de, johnstul@...ibm.com, zippel@...ux-m68k.org,
	andrea@...e.de
Subject: Re: [patch 4/9] Remove the TSC synchronization on SMP machines

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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