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Date: Fri, 10 Dec 2004 22:24:51 +0100
From: Ingo Molnar <mingo@elte.hu>
To: Mark_H_Johnson@raytheon.com
Cc: Amit Shah <amit.shah@codito.com>,
       Karsten Wiese <annabellesgarden@yahoo.de>, Bill Huey <bhuey@lnxw.com>,
       Adam Heath <doogie@debian.org>, emann@mrv.com,
       Gunther Persoons <gunther_persoons@spymac.com>,
       "K.R. Foley" <kr@cybsft.com>, linux-kernel@vger.kernel.org,
       Florian Schmidt <mista.tapas@gmx.net>,
       Fernando Pablo Lopez-Lezcano <nando@ccrma.Stanford.EDU>,
       Lee Revell <rlrevell@joe-job.com>, Rui Nuno Capela <rncbc@rncbc.org>,
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       Thomas Gleixner <tglx@linutronix.de>,
       Michal Schmidt <xschmi00@stud.feec.vutbr.cz>
Subject: Re: [patch] Real-Time Preemption, -RT-2.6.10-rc2-mm3-V0.7.32-15
Message-ID: <20041210212451.GF5864@elte.hu>
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* Mark_H_Johnson@raytheon.com <Mark_H_Johnson@raytheon.com> wrote:

> [...] I also had several cases where I "triggered" a trace but no
> output - I assume those are related symptoms. For example:
> 
> # ./cpu_delay 0.000100
> Delay limit set to 0.00010000 seconds
> calibrating loop ....
> time diff= 0.504598 or 396354830 loops/sec.
> Trace activated with 0.000100 second delay.
> Trace triggered with 0.000102 second delay. [not recorded]
> Trace activated with 0.000100 second delay.
> Trace triggered with 0.000164 second delay. [not recorded]

is the userspace delay measurement nested inside the kernel-based
method? I.e. is it something like:

	gettimeofday(0,1);
	timestamp1 = cycles();

	... loop some ...

	timestamp2 = cycles();
	gettimeofday(0,0);

and do you get 'unreported' latencies in such a case too? If yes then
that would indeed indicate a tracer bug. But if the measurement is done
like this:

	gettimeofday(0,1);
	timestamp1 = cycles();

	... loop some ...

	gettimeofday(0,0);		// [1]
	timestamp2 = cycles();		// [2]

then a delay could get inbetween [1] and [2].

OTOH if the 'loop some' time is long enough then the [1]-[2] window is
too small to be significant statistically, while your logs show a near
50% 'miss rate'.

	Ingo