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

[Darren Hart <dvhart@linux.intel.com>]

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@infradead.org>  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 ? 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 - 
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.

Shall I respin the patch to reflect that?


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