Subject: Re: [PATCH 2/3] dyntick - Fix lost tick calculation in timer pm.c
From: john stultz <johnstul@us.ibm.com>
To: Thomas Schlichter <thomas.schlichter@web.de>
Cc: vatsa@in.ibm.com, linux-kernel@vger.kernel.org, akpm@osdl.org,
       Con Kolivas <kernel@kolivas.org>, ck list <ck@vds.kolivas.org>
In-Reply-To: <200509022218.04796.thomas.schlichter@web.de>
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	 <200509030145.18368.kernel@kolivas.org> <20050902172504.GB4650@in.ibm.com>
	 <200509022218.04796.thomas.schlichter@web.de>
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Date: Fri, 02 Sep 2005 14:21:36 -0700
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On Fri, 2005-09-02 at 22:18 +0200, Thomas Schlichter wrote:
> 2. Can we really assure that the monotonic clock is still monotonic?
> I think with your new code we estimate the monotonic clock value and the 
> offset_last at the last tick.
> But if we underestimate monotonic_base or overestimate offset_last (even 
> simply by rounding errors), the time will make a small step backwards with 
> the value-update.
> And as far as I understand the monotonic clock its not that bad if it drifts a 
> bit, but it is really bad if time makes steps backward...
> 
> But maybe you can show me that I am wrong with my second point.
> I hope I don't bother you too much with this kind of stuff...
> 
>   Thomas
> 
> P.S.: I CC'd John because he knows the monotonic clock better than I do... :-)


Thanks Thomas, that's a good catch. Since monotonic_clock has no real
notion of interrupt edges (it was designed to be constant regardless if
we miss ticks), I would keep accumulating the full inter-tick intervals
converted to usecs into the monotonic_base.

thanks
-john


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