Re: [PATCH 2/2] sched: allow users with rtprio rlimit to change fromSCHED_IDLE policy

[Darren Hart]

On 02/23/2011 08:00 AM, Peter Zijlstra wrote:
> 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.

I was just thinking in terms of POLICY, not priority or nice level. I'll 
do some experimenting and see where the limits are.

> > 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.

Will do.

--
Darren Hart
Intel Open Source Technology Center
Yocto Project - Linux Kernel
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