Re: 2.6.11-rc3-mm2

[Matt Mackall <mpm@selenic.com>]

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.