Subject: Re: report a bug about sched_rt
From: Peter Zijlstra <peterz@infradead.org>
To: sen wang <wangsen.linux@gmail.com>
Cc: mingo@elte.hu, akpm@linux-foundation.org, kernel@kolivas.org,
       npiggin@suse.de, arjan@infradead.org,
       linux-arm-kernel@lists.arm.linux.org.uk, linux-kernel@vger.kernel.org
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Date: Fri, 24 Jul 2009 15:14:50 +0200
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Don't top post -- again!

On Fri, 2009-07-24 at 21:04 +0800, sen wang wrote:
> Linux is used in many fieldes. SCHED_OTHER tasks is important to
> embedded system.

Irrelevant.

> if there is a running state task（a realtime task）, how can we
> shcedule the idle task up?

Because it ran out of bandwidth.

> It is ridiculous！
> 
> since the throttle has a bug, why not fix it?

It doesn't have a bug, therefore I won't fix it.

The throttle limits the RT tasks to a bandwidth w of u/p.
Since real-time scheduling is about determinism a maximum bandwidth
larger than the minimum bandwidth specified by w is useless since it
cannot be relied upon.

Therefore we don't run RT tasks beyond their bandwidth limit.

Go read up on scheduling theory.

Now you might want a bandwidth of 100% for your RT application (not
something I can recommend for the overall health of your machine) in
which case you're free to change this setting:

  echo -1 > /proc/sys/kernel/sched_rt_runtime_us

Should do that for you. Also read:

  Documentation/scheduler/sched-rt-group.txt


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