Re: [PATCH 2/2] sched: allow users with rtprio rlimit to changefrom SCHED_IDLE policy

[Peter Zijlstra]

On Wed, 2011-02-23 at 07:52 -0800, Darren Hart wrote:
> On 02/23/2011 03:17 AM, Peter Zijlstra wrote:
> > On Wed, 2011-02-23 at 12:13 +0100, Ingo Molnar wrote:
> >> * Peter Zijlstra<peterz@xxxxxxxxxxxxx>  wrote:
> >>
> >>> On Tue, 2011-02-22 at 13:04 -0800, Darren Hart wrote:
> >>>> As it stands, users with rtprio rlimit permissions can change their policy from
> >>>> SCHED_OTHER to SCHED_FIFO and back. They can change to SCHED_IDLE, but not back
> >>>> to SCHED_FIFO. If they have the rtprio permission, they should be able to. Once
> >>>> in SCHED_FIFO, they could go back to SCHED_OTHER. This patch allows users with
> >>>> rtprio permission to change out of SCHED_IDLE.
> >>>>
> >>>
> >>> Ingo, can you remember the rationale for this?
> >>>
> >>> The fact is that SCHED_IDLE is very near nice-20, and we can do:
> >>>
> >>> peterz@twins:~$ renice 5 -p $$
> >>> 1867: old priority 0, new priority 5
> >>> peterz@twins:~$ renice 0 -p $$
> >>> 1867: old priority 5, new priority 0
> >>>
> >>> Which would suggest that we should be able to return to SCHED_OTHER
> >>> RLIMIT_NICE-20.
> >>
> >> I dont remember anything subtle there - most likely we just forgot about that spot
> >> when adding RLIMIT_RTPRIO support.
> >
> > Ah, I was arguing we should allow it regardless of RLIMIT_RTPRIO, based
> > on RLIMIT_NICE, it is after all a change to SCHED_OTHER, not
> > SCHED_FIFO/RR.
> 
> So we need an OR test of RLIMIT_NICE | RLIMIT_RTPRIO ? 

Just RLIMIT_NICE I think.

> The reason I keep 
> coming back to RTPRIO is it allows the user to change to 
> SCHED_(FIFO|RR), and from there they can change to anything they want - 

Hmm,. is that so? I would think that even if you're SCHED_FIFO changing
back to SCHED_OTHER ought to make you respect RLIMIT_NICE.

That is, even if you're a SCHED_FIFO-1 task due to RLIMIT_RTPRIO, when
you switch back to SCHED_OTHER I would expect you not to be able to
switch to a lower nice than RLIMIT_NICE-20.

Now, if this isn't actually so I think we ought to make it so.

> so why force two steps? Perhaps the argument is to keep the meaning of 
> the RLIMITs precise, and if you want to go from IDLE->OTHER you had 
> better properly set RLIMIT_NICE - maybe I just convinced myself.

Right

> Shall I respin the patch to reflect that?

Please.
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