linux-kernel - Re: [PATCH 6/6] sched: disabled rt-bandwidth by default

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Message-ID: <20080828110923.GL26610@one.firstfloor.org>
Date:	Thu, 28 Aug 2008 13:09:23 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Nick Piggin <nickpiggin@...oo.com.au>,
	Andi Kleen <andi@...stfloor.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-kernel@...r.kernel.org,
	Stefani Seibold <stefani@...bold.net>,
	Dario Faggioli <raistlin@...ux.it>,
	Max Krasnyansky <maxk@...lcomm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 6/6] sched: disabled rt-bandwidth by default

> Even if the system has multiple CPUs, and even if just a single CPU is 
> fully utilized by an RT task, without the rt-limit the system will still 
> lock up in practice due to various other factors: workqueues and tasks 
> being 'stuck' on CPUs that host an RT hog. 

The load balancer will not notice that a particular CPU is busy
with real time tasks?

> While there's obviously CPU 
> time available on other CPUs, you cannot run 'top', the desktop will 
> freeze, work flows of the system can be stuck, etc, etc..

I had such a situation at least once in the past (not due
run away RT but due a kernel bug) and even with 2 out of 4 CPUs blocked 
the system was still quite usable. top/kill definitely worked.  The system 
didn't have a desktop, but I didn't notice many problems in shell use. 
Ok it's just one sample.

That said I don't think having such a limit by default is a bad idea actually.

Just handling it in the scheduler anyways is also probably good because
it can happen even due to other issues than just run away RT tasks.

-Andi
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