linux-kernel - Re: 1 RT task blocks 4-core machine ?

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Message-ID: <1286372045.2144.180.camel@laptop>
Date:	Wed, 06 Oct 2010 15:34:05 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Tommaso Cucinotta <tommaso.cucinotta@...up.it>
Cc:	Dhaval Giani <dhaval.giani@...il.com>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Dario Faggioli <faggioli@...dalf.sssup.it>,
	Fabio Checconi <fabio@...dalf.sssup.it>,
	linux-kernel@...r.kernel.org
Subject: Re: 1 RT task blocks 4-core machine ?

On Tue, 2010-10-05 at 00:26 +0200, Tommaso Cucinotta wrote:
> A possible explanation might be that the CFS load balancing logic sees
> my only active task (e.g., the ssh server or shell etc.) as running
> alone on its core, and does not detect that it is inhibited to actually
> run due to RT tasks on the same core. Therefore, it will not migrate
> the task to the free cores. Does this explanation make sense
> or is it completely wrong ?

Possibly, its got some logic to detect this but maybe it gets confused
still, in particular look at the adaptive cpu_power in
update_cpu_power() and calling functions.

> Also, I'd like to hear whether this is considered the "normal/desired"
> behavior of intermixing RT and non-RT tasks. 

Pegging a cpu using sched_fifo/rr pretty much means you get to keep the
pieces, if it works nice, if you can make it work better kudos, but no
polling from sched_fifo/rr is not something that is considered sane for
the general health of your system.
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