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

[Peter Zijlstra <peterz@infradead.org>]

On Wed, 2011-02-23 at 13:28 -0800, Darren Hart wrote:
> From: Darren Hart <dvhart@linux.intel.com>
> Date: Thu, 17 Feb 2011 15:37:07 -0800
> Subject: [PATCH] sched: allow users with sufficient RLIMIT_NICE to change from SCHED_IDLE policy
> 
> The current scheduler implementation returns -EPERM when trying to change from
> SCHED_IDLE to SCHED_OTHER or SCHED_BATCH.


>  Since SCHED_IDLE is considered to be
> equivalent to a nice 20, 

Well, its not quite equivalent, its actually 5 times lighter still and
the preemption behaviour is slightly different as you've found ;-)

> changing to another policy should be allowed provided
> the RLIMIT_NICE is accounted for.
> 
> This patch allows the following test-case to pass with RLIMIT_NICE=40, but still
> fail with RLIMIT_NICE=10 when the calling process is run from a typical shell
> (nice 0, or 20 in rlimit terms).

> 
> Signed-off-by: Darren Hart <dvhart@linux.intel.com>
> CC: Peter Zijlstra <peterz@infradead.org>
> CC: Ingo Molnar <mingo@elte.hu>
> CC: Richard Purdie <richard.purdie@linuxfoundation.org>
> ---
>  kernel/sched.c |   11 +++++++----
>  1 files changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/sched.c b/kernel/sched.c
> index 18d38e4..9bf6284 100644
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -4822,12 +4822,15 @@ recheck:
>                             param->sched_priority > rlim_rtprio)
>                                 return -EPERM;
>                 }
> +
>                 /*
> +                * Treat SCHED_IDLE as nice 20. Only allow a switch to
> +                * SCHED_NORMAL if the RLIMIT_NICE would normally permit it.
>                  */
> +               if (p->policy == SCHED_IDLE && policy != SCHED_IDLE) {
> +                       if (!can_nice(p, TASK_NICE(p)))
> +                               return -EPERM;
> +               }
>  
>                 /* can't change other user's priorities */
>                 if (!check_same_owner(p)) 

Looks fine, thanks!