linux-kernel - [RFC] [rt-tests] change to cyclictest behavior

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Date:	Wed, 6 Jan 2010 13:04:00 -0600
From:	Clark Williams <williams@...hat.com>
To:	RT <linux-rt-users@...r.kernel.org>
Cc:	LKML <linux-kernel@...r.kernel.org>
Subject: [RFC] [rt-tests] change to cyclictest behavior

RT-ers,

I have a problem with the way cyclictest sets up measurement threads,
but before I went and changed things I thought I would ask if people
cherished this particular behavior. 

Currently, when cyclictest is run with multiple threads (i.e. -t
option) it distributes both the sample interval and the realtime
priority by adding the 'distance' parameter to the interval and
decrementing the priority by one. This means if you have a distance of
500us (default), a specified RT priority of 95 and start four threads,
they will be started with the following parameters:

$ cyclictest -t4 -p95

Will give you:

thread		priority	sample interval
0		95		500
1		94		1000
2		93		1500
3		92		2000

What I'd like to do is modify this logic so that when '-a' (affinity) is
specified, the priority and sample interval will not be altered. I
don't think there's any point in distributing the priority's and
sample intervals when the measurement threads are pinned to their own
CPU. 

So:

$ cyclictest -t4 -p95 -a

Would have each thread at SCHED_FIFO 95 and a sample interval of 500us.

Note that this behavior also occurs when the histogram (-h) option is
specified).

Thoughts?

Clark

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