Date: Mon, 7 Jan 2008 12:09:04 +0100
From: Ingo Molnar <mingo@elte.hu>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Michal Schmidt <mschmidt@redhat.com>, linux-kernel@vger.kernel.org,
       "Eric W. Biederman" <ebiederm@xmission.com>,
       Jon Masters <jcm@redhat.com>,
       Satoru Takeuchi <takeuchi_satoru@jp.fujitsu.com>,
       Peter Zijlstra <a.p.zijlstra@chello.nl>
Subject: Re: [PATCH] kthread: always create the kernel threads with normal
	priority
Message-ID: <20080107110904.GB28481@elte.hu>
References: <20071217234314.540b59bd@hammerfall> <20071222013021.db2528cb.akpm@linux-foundation.org> <20080107110603.09b72450@brian.englab.brq.redhat.com> <20080107022513.3ac05734.akpm@linux-foundation.org>
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> > 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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