Re: [patch, 2.6.11-rc2] sched: RLIMIT_RT_CPU_RATIO feature

[Ingo Molnar]

* Nick Piggin <nickpiggin@xxxxxxxxxxxx> wrote:

> Well in the context of a multi user system, this really is a
> privileged operation. Witness: a normal user isn't even allowed to
> raise the nice priority of a normal task. Note that I think everyone
> agrees here, but I'm just repeating the point.

i've seen this argument repeated a number of times, but i'd like to
point out that with the rlimit set to a sane value, a user can do 'less
damage' to the system via SCHED_FIFO than it could do via nice--20!

negative nice levels are a guaranteed way to monopolize the CPU. 
SCHED_FIFO with throttling could at most be used to 'steal' CPU time up
to the threshold. Also, if a task 'runs away' in SCHED_FIFO mode it will
be efficiently throttled. While if it 'runs away' in nice--20 mode, it
will take away 95+% of the CPU time quite agressively. Furthermore, more
nice--20 tasks will do much more damage (try thunk.c at nice--20!),
while more throttled SCHED_FIFO tasks only do damage to their own class
- the guaranteed share of SCHED_OTHER tasks (and privileged RT tasks) is
not affected.

so while it is true that in terms of priorities, throttled SCHED_FIFO
trumps all SCHED_OTHER tasks, but in the "potential damage" sense,
"throttled real-time" is less of a privilege than "nice--20".

	Ingo
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