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Subject: Re: [patchlet] sched: fix rt throttle runtime borrowing
From: Mike Galbraith <efault@gmx.de>
To: Yong Zhang <yong.zhang0@gmail.com>
Cc: LKML <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>, Ingo Molnar <mingo@elte.hu>
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Date: Mon, 07 Mar 2011 10:33:34 +0100
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On Mon, 2011-03-07 at 17:11 +0800, Yong Zhang wrote:
> On Mon, Mar 7, 2011 at 4:21 PM, Mike Galbraith <efault@gmx.de> wrote:
> > Greetings,
> >
> > The RT throttle leaves a bit to be desired as a protection mechanism.
> > With default settings, the thing won't save your bacon if you start a
> > single hog as RT on SMP box, or if your normally sane app goes nuts.
> >
> > With the below, my box will limp along so I can kill the RT hog.  May
> > not be the best solution, but works for me.. modulo bustage I haven't
> > noticed yet of course.
> >
> > sched: fix rt throttle runtime borrowing
> >
> > If allowed to borrow up to rt_period, the throttle has no effect on an out
> > of control RT task, allowing it to consume 100% CPU indefinitely, blocking
> > system critical SCHED_NORMAL threads indefinitely.
> 
> Yep.
> I think it's helpful.

Well, it does prevent complete death, but you have to be pretty darn
attentive to notice that the patient is still technically alive ;-)

As such, turning borrowing off by default, and making borrowing up to
within a micron of 100% CPU an opt-in feature likely makes more sense.

	-Mike