[PATCH] sched: allow users with sufficient RLIMIT_NICE to change from SCHED_IDLE policy

[Darren Hart]

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, 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).

int main()
{
	int ret;
	struct sched_param sp;
	sp.sched_priority = 0;

	/* switch to SCHED_IDLE */
	ret = sched_setscheduler(0, SCHED_IDLE, &sp);
	printf("setscheduler IDLE: %d\n", ret);
	if (ret) return ret;

	/* switch back to SCHED_OTHER */
	ret = sched_setscheduler(0, SCHED_OTHER, &sp);
	printf("setscheduler OTHER: %d\n", ret);

	return ret;
}

$ ulimit -e
40
$ ./test
setscheduler IDLE: 0
setscheduler OTHER: 0

$ ulimit -e 10
$ ulimit -e
10
$ ./test
setscheduler IDLE: 0
setscheduler OTHER: -1

Signed-off-by: Darren Hart <dvhart@xxxxxxxxxxxxxxx>
CC: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CC: Ingo Molnar <mingo@xxxxxxx>
CC: Richard Purdie <richard.purdie@xxxxxxxxxxxxxxxxxxx>
---
 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;
 		}
+
 		/*
-		 * Like positive nice levels, dont allow tasks to
-		 * move out of SCHED_IDLE either:
+		 * 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)
-			return -EPERM;
+		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))
-- 
1.7.1


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