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 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@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 ?
>
> 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