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

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Date:	Mon, 7 Jan 2008 09:29:56 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Ingo Molnar <mingo@...e.hu>
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

On Mon, 7 Jan 2008 12:09:04 +0100 Ingo Molnar <mingo@...e.hu> wrote:

> 
> > > 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.

Is that "total RT CPU time" or "elapsed time since last schedule()"?

If the former, it is not useful for this problem.

> 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)

Doesn't this make the RT task non-RT?  Would need to understand more
details to tell.

> 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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