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Date: Wed, 9 Sep 2009 22:32:16 +0200
From: Luigi Rizzo <rizzo@iet.unipi.it>
To: John Baldwin <jhb@freebsd.org>
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Cc: freebsd-stable@freebsd.org
Subject: Re: incorrect usleep/select delays with HZ > 2500
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On Wed, Sep 09, 2009 at 01:24:53PM -0400, John Baldwin wrote:
> On Tuesday 08 September 2009 5:01:00 pm Luigi Rizzo wrote:
> > On Tue, Sep 08, 2009 at 01:08:23PM -0400, John Baldwin wrote:
> > > On Sunday 06 September 2009 11:51:54 am Luigi Rizzo wrote:
> > > > [Note 3] the TSC frequency is computed reading the tsc around a
> > > >         call to DELAY(1000000) and assuming that the i8254 runs
> > > >         at the nominal rate, 1.193182 MHz.
> > > >         From tests I have made, the measurement in init_TSC() returns
> > > >         a large error when HZ is large, whereas repeating the 
> measurement
> > > >         at a later time returns a much more reliable value.
...
> > > Try disabling legacy USB support in the BIOS to see if an SMI# is firing 
> > > during the DELAY() causing the TSC freq to be too high.  I have seen the 
> USB 
> > > legacy support cause this in other machines.
> > 
> > Thanks, will try tomorrow.
> > Would this explain the measurement becomes better as we get
> > further into the initialization, and why high HZ values affect
> > the measurement ?
> 
> It would explain why it gets better later since the uhci(4), ohci(4) and 
> ehci(4) drivers disable the SMI# interrupts while attaching to the 
> controllers.

ok makes sense -- if the SMI interrupts last longer than 1/HZ
they will cause us to miss one or more wraps of the i8254 in
$ARCH/isa/clock.c::DELAY(). I suppose with a few measurements
at different HZ values and some back of the envelope calculations
one could even determine estimate the frequency and duration
of those SMI interrupts!

thanks
luigi