Re: SCHED_DEADLINE, sched_getscheduler(), and sched_getparam()

[Peter Zijlstra <peterz@infradead.org>]

On Mon, May 12, 2014 at 09:42:44PM +0200, Michael Kerrisk (man-pages) wrote:
> > Hmm,.. maybe. Can we still change this? Again, maybe, there's not really
> > that much userspace that relies on this.
> 
> I think the sched_getparam() change is worthwhile (and the patches 
> could (should?) be marked for -stable). I suspect there's no user
> space that relies on the current SCHED_DEADLINE behavior, and it's 
> worth avoiding the above breakage for sched_getparam(). I'd be 
> inclined to leave sched_getscheduler() as is: there's arguments 
> either way for how it should behave.
> 
> > In any case, the way I read the little there is on getparam() it seems
> > to imply the only case where it does make sense to call it at all is
> > when sched_getscheduler() returns either SCHED_FIFO or SCHED_RR.
> 
> (Yes, that's my understanding too.)

Something like so then, it encodes that reading explicitly.

---
Subject: sched: Change sched_getparam() behaviour vs SCHED_DEADLINE
From: Peter Zijlstra <peterz@infradead.org>
Date: Mon May 12 22:22:47 CEST 2014

The way we read POSIX one should only call sched_getparam() when
sched_getscheduler() returns either SCHED_FIFO or SCHED_RR.

Given that we currently return sched_param::sched_priority=0 for all
others, extend the same behaviour to SCHED_DEADLINE.

Requested-by: Michael Kerrisk <mtk.manpages@gmail.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
---
 kernel/sched/core.c |    9 +++------
 1 file changed, 3 insertions(+), 6 deletions(-)

Index: linux-2.6/kernel/sched/core.c
===================================================================
--- linux-2.6.orig/kernel/sched/core.c
+++ linux-2.6/kernel/sched/core.c
@@ -3759,7 +3759,7 @@ SYSCALL_DEFINE1(sched_getscheduler, pid_
  */
 SYSCALL_DEFINE2(sched_getparam, pid_t, pid, struct sched_param __user *, param)
 {
-	struct sched_param lp;
+	struct sched_param lp = { .sched_priority = 0 };
 	struct task_struct *p;
 	int retval;
 
@@ -3776,11 +3776,8 @@ SYSCALL_DEFINE2(sched_getparam, pid_t, p
 	if (retval)
 		goto out_unlock;
 
-	if (task_has_dl_policy(p)) {
-		retval = -EINVAL;
-		goto out_unlock;
-	}
-	lp.sched_priority = p->rt_priority;
+	if (task_has_rt_policy(p))
+		lp.sched_priority = p->rt_priority;
 	rcu_read_unlock();
 
 	/*