[PATCH] sched/core: sched_getattr returning consistent sched_priority

[Alessio Balsini]

Always initialise the sched_priority field of the sched_attr struct
returned by sched_getattr().
The sched_getattr() syscall takes care of returning a sched_attr
structure updated with the current scheduling "attributes" of the
requested thread. This syscall function is dual to sched_setattr() that,
instead, assigns the given values to the specified thread.

sched_setattr(), as described in the documentation, imposes that
whenever a thread switches to any non-RT scheduling policy, rt_priority
must be 0. This check is performed by __sched_setscheduler() which, in
case of a negative result, ignores the request and returns -EINVAL (*).
Thus, when calling sched_getattr(), the user would expect that non-RT
threads will have sched_priority equal to 0 and RT threads 1 <=
sched_priority <= 99.
As a result, the sched_priority field should always be specified by
sched_getattr() with the rt_priority of the thread, whose value is
coherent thanks to (*).

Since the RT policy check is dropped, the condition to update sched_nice
is made explicit with the introduced task_has_fair_policy().
The parameters associated with FAIR and DL tasks can be inconsistent for
the non-corresponding scheduling classes, and this behaviour, left
unchanged, is correct since it does not violate the documentation.

Moreover, __getparam_dl(), the function that takes care of filling the
the sched_attr parameters associated with DL tasks, updates also
sched_priority. Here, the sched_priority field is out of scope and is
removed.
This inaccuracy was introduced in 06a76fe08d4d, that moved the function
from core.c to deadline.c. Before that, it was making more sense to
access sched_priority, either if the function name __getparam_dl() was
misleading.

Signed-off-by: Alessio Balsini <alessio.balsini@xxxxxxx>
Reviewed-by: Patrick Bellasi <patrick.bellasi@xxxxxxx>
Reviewed-by: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Tommaso Cucinotta <tommaso.cucinotta@xxxxxxxxxxxxxxx>
Cc: Luca Abeni <luca.abeni@xxxxxxxxxxxxxxx>
---
 kernel/sched/core.c     | 5 ++---
 kernel/sched/deadline.c | 1 -
 kernel/sched/sched.h    | 5 +++++
 3 files changed, 7 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 75554f3..174d611 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4606,11 +4606,10 @@ SYSCALL_DEFINE4(sched_getattr, pid_t, pid, struct sched_attr __user *, uattr,
 	attr.sched_policy = p->policy;
 	if (p->sched_reset_on_fork)
 		attr.sched_flags |= SCHED_FLAG_RESET_ON_FORK;
+	attr.sched_priority = p->rt_priority;
 	if (task_has_dl_policy(p))
 		__getparam_dl(p, &attr);
-	else if (task_has_rt_policy(p))
-		attr.sched_priority = p->rt_priority;
-	else
+	else if (task_has_fair_policy(p))
 		attr.sched_nice = task_nice(p);
 
 	rcu_read_unlock();
diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 2473736..0ad9cfd 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -2502,7 +2502,6 @@ void __getparam_dl(struct task_struct *p, struct sched_attr *attr)
 {
 	struct sched_dl_entity *dl_se = &p->dl;
 
-	attr->sched_priority = p->rt_priority;
 	attr->sched_runtime = dl_se->dl_runtime;
 	attr->sched_deadline = dl_se->dl_deadline;
 	attr->sched_period = dl_se->dl_period;
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index b19552a2..8f410fc 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -146,6 +146,11 @@ static inline bool valid_policy(int policy)
 		rt_policy(policy) || dl_policy(policy);
 }
 
+static inline int task_has_fair_policy(struct task_struct *p)
+{
+	return fair_policy(p->policy);
+}
+
 static inline int task_has_rt_policy(struct task_struct *p)
 {
 	return rt_policy(p->policy);
-- 
2.7.4