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Date: Thu, 2 Dec 2004 14:40:37 +0100
From: Florian Schmidt <mista.tapas@gmx.net>
To: Ingo Molnar <mingo@elte.hu>
Cc: Rui Nuno Capela <rncbc@rncbc.org>, linux-kernel@vger.kernel.org,
       Lee Revell <rlrevell@joe-job.com>, mark_h_johnson@raytheon.com,
       "K.R. Foley" <kr@cybsft.com>, Bill Huey <bhuey@lnxw.com>,
       Adam Heath <doogie@debian.org>, Thomas Gleixner <tglx@linutronix.de>,
       Michal Schmidt <xschmi00@stud.feec.vutbr.cz>,
       Fernando Pablo Lopez-Lezcano <nando@ccrma.stanford.edu>,
       Karsten Wiese <annabellesgarden@yahoo.de>,
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       Shane Shrybman <shrybman@aei.ca>, Amit Shah <amit.shah@codito.com>,
       Esben Nielsen <simlo@phys.au.dk>, Andrew Morton <akpm@osdl.org>
Subject: Re: [patch] Real-Time Preemption, -RT-2.6.10-rc2-mm3-V0.7.31-19
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On Thu, 2 Dec 2004 14:10:02 +0100
Ingo Molnar <mingo@elte.hu> wrote:

> 
> * Florian Schmidt <mista.tapas@gmx.net> wrote:
> 
> > Hmm, i wonder if there's a way to detect non RT behaviour in jackd
> > clients. I mean AFAIK the only thing allowed for the process callback
> > of on is the FIFO it waits on to be woken, right? Every other sleeping
> > is to be considered a bug. 
> 
> there's such a feature in -RT kernels. If a user process calls:
> 
> 	gettimeofday(1,1);
> 
> then the kernel turns 'atomic mode' on. To turn it off, call:
> 
> 	gettimeofday(1,0);
> 
> while in atomic-mode, any non-atomic activity (scheduling) will produce
> a kernel message and a SIGUSR2 sent to the offending process (once,
> atomic mode has to be re-enabled again for the next message). Preemption
> by a higher-prio task does not trigger a message/signal.
> 
> If you run the client under gdb you should be able to catch the SIGUSR2
> signal and then you can see the offending code's backtrace via 'bt'.

Ok, so if i want to find out whether a client violates the RT
constraints for its process callback i would have to add a call to
gettimeofday(1,1) at the start of the process callback and
gettimeofday(1,0) at the end.

Everything which causes a reschedule inbetween will then cause SIGUSR2
to be sent to the client for which i could either add a signal handler
in the client or just use gdb to get notified of it. 

Cool!

Flo