Date: Fri, 11 Feb 2005 01:27:15 -0800
From: Matt Mackall <mpm@selenic.com>
To: Ingo Molnar <mingo@elte.hu>
Cc: Paul Davis <paul@linuxaudiosystems.com>,
       Peter Williams <pwil3058@bigpond.net.au>,
       Nick Piggin <nickpiggin@yahoo.com.au>, Chris Wright <chrisw@osdl.org>,
       "Jack O'Quin" <jack.oquin@gmail.com>, Andrew Morton <akpm@osdl.org>,
       Christoph Hellwig <hch@infradead.org>, linux-kernel@vger.kernel.org,
       Con Kolivas <kernel@kolivas.org>, rlrevell@joe-job.com
Subject: Re: 2.6.11-rc3-mm2
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On Fri, Feb 11, 2005 at 10:04:19AM +0100, Ingo Molnar wrote:
> 
> * Matt Mackall <mpm@selenic.com> wrote:
> 
> > So the comparison boils down to putting a magic gid in a sysfs
> > file/module parameter or setting an rlimit with standard tools (PAM,
> > etc). I'm really boggled that anyone could prefer the former,
> > especially since we had almost this exact debate over what became the
> > mlock rlimit!
> 
> the big difference to mlock is that for mlock there _is_ a _limit_. For
> RT scheduling the priority is _NOT_ a _limit_. Okay? So you give the
> false pretense of this being some kind of resource 'limit', while in
> fact allowing SCHED_FIFO prio 1 alone enables unprivileged users to lock
> up the system.
>
> so i could agree with RLIMIT_NICE (which _is_ a limit), but
> RLIMIT_RTPRIO sends the wrong message. The proper rlimit would be
> RLIMIT_RT_CPU (the patch i did).

It's not a perfect fit, I'll readily agree.

But consider this: with RLIMIT_RTPRIO, I can restrict a user to the
lowest N RT priorities. Then at N+1, I have an RT watchdog taking care
of runaways, tickled by a SCHED_NORMAL task. So it can still be looked
at as a meaningful limit, just a bit different from the others.

The RT LSM gives full CAP_SYS_NICE out, so there's no way to guarantee
that the watchdog has higher priority.

-- 
Mathematics is the supreme nostalgia of our time.
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