linux-kernel - Re: [PATCH] kthread: always create the kernel threads with normal priority

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Message-ID: <20080107110904.GB28481@elte.hu>
Date:	Mon, 7 Jan 2008 12:09:04 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Michal Schmidt <mschmidt@...hat.com>, linux-kernel@...r.kernel.org,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Jon Masters <jcm@...hat.com>,
	Satoru Takeuchi <takeuchi_satoru@...fujitsu.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH] kthread: always create the kernel threads with normal
	priority

> > This causes a practical problem. When a runaway real-time task is 
> > eating 100% CPU and we attempt to put the CPU offline, sometimes we 
> > block while waiting for the creation of the highest-priority 
> > "kstopmachine" thread.

sched-devel.git has new mechanisms against runaway RT tasks. There's a 
new RLIMIT_RTTIME rlimit - if an RT task exceeds that rlimit then it is 
sent SIGXCPU.

there's also a new group scheduling extension that is driven via a 
sysctl:

  /proc/sys/kernel/sched_rt_ratio

this way if a user has a runaway RT task, other users (and root) will 
still have some CPU time left. (in Peter's latest patchset that is 
replaced via rt_runtime_ns - but this is a detail)

so instead of the never-ending arms race of kernel thread priorities 
against RT task priorities, we are going towards making RT tasks safer 
on a policy level.

	Ingo
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