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Date: Thu, 25 Nov 2004 18:14:30 +0100
From: Ingo Molnar <mingo@elte.hu>
To: Esben Nielsen <simlo@phys.au.dk>
Cc: linux-kernel@vger.kernel.org
Subject: Re: Priority Inheritance Test (Real-Time Preemption)
Message-ID: <20041125171430.GA25886@elte.hu>
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* Esben Nielsen <simlo@phys.au.dk> wrote:

> 
> On Thu, 25 Nov 2004, Ingo Molnar wrote:
> 
> > [...] 
> > there's one thing i noticed, now that the blocker device is in the
> > kernel, you have to be really careful to compile the userspace loop()
> > code via the same gcc flags as the kernel did. Minor differences in
> > compiler options can skew the timing calibration.
> > 
> > but any such bug should at most cause a linear deviation via a constant
> > factor multiplication, while the data shows a systematic nonlinear
> > transformation.
> > 
> -g -Wall -O2 was on in userspace.

you can check the gcc options the kernel used via the
drivers/char/.blocker.o.cmd file. Mine has (only the
performance-relevant flags):

 -fno-strict-aliasing -fno-common -O2 -fomit-frame-pointer -msoft-float
 -mpreferred-stack-boundary=2 -fno-unit-at-a-time -march=pentium3
 -mregparm=3

> > [...] 
> > yeah, i agree that this has to be further investigated. What type of box
> > did you test it on - UP or SMP? (SMP scheduling of RT tasks only got
> > fully correct in the very latest -31-7 kernel.)
> > 
> UP, PIII 697.143 Mhz

ok - some of the fixes affect UP too, but with less likelyhood. Might be
worth a try though.

	Ingo