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

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Message-ID: <520f0cf11001061139j2af13403qfcbf567647bdfaa8@mail.gmail.com>
Date:	Wed, 6 Jan 2010 20:39:25 +0100
From:	John Kacur <jkacur@...hat.com>
To:	Clark Williams <williams@...hat.com>
Cc:	RT <linux-rt-users@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] [rt-tests] change to cyclictest behavior

On Wed, Jan 6, 2010 at 8:04 PM, Clark Williams <williams@...hat.com> wrote:
> 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?
>

Seems reasonable to me. Maybe it would also be nice to have a flag to
get the old behaviour back even with -a?

John
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