[PATCH v2 01/21] sched: Reject SCHED_FLAG_KEEP_PARAMS before reserving deadline bandwidth

[Mathieu Desnoyers]

__sched_setscheduler() returns -EINVAL for a SCHED_FLAG_KEEP_PARAMS
request that would change scheduling class, after sched_dl_overflow()
has reserved the bandwidth of the request, and nothing gives it back.
Switching a SCHED_OTHER task to SCHED_DEADLINE that way fails and leaks
the runtime that sched_setattr() filled in from the task's slice, up to
100ms with a custom one, into the root domain's total_bw. Repeated, it
makes the root domain refuse every deadline task, and refuse to offline
its last CPU.

Reserve the bandwidth after that check, last of the checks. A refused
request also leaves sched_reset_on_fork as it was again.

Fixes: 8a70415351ba ("sched: Reject SCHED_FLAG_KEEP_PARAMS when it would change scheduling class")
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
---
 kernel/sched/syscalls.c | 24 +++++++++++++-----------
 1 file changed, 13 insertions(+), 11 deletions(-)

diff --git a/kernel/sched/syscalls.c b/kernel/sched/syscalls.c
index 3fa447aabe7f..f0b7b2d52614 100644
--- a/kernel/sched/syscalls.c
+++ b/kernel/sched/syscalls.c
@@ -655,17 +655,6 @@ int __sched_setscheduler(struct task_struct *p,
 		goto recheck;
 	}
 
-	/*
-	 * If setscheduling to SCHED_DEADLINE (or changing the parameters
-	 * of a SCHED_DEADLINE task) we need to check if enough bandwidth
-	 * is available.
-	 */
-	if ((dl_policy(policy) || dl_task(p)) && sched_dl_overflow(p, policy, attr)) {
-		retval = -EBUSY;
-		goto unlock;
-	}
-
-	p->sched_reset_on_fork = reset_on_fork;
 	oldprio = p->prio;
 
 	newprio = __normal_prio(policy, attr->sched_priority, attr->sched_nice);
@@ -699,6 +688,19 @@ int __sched_setscheduler(struct task_struct *p,
 		queue_flags |= DEQUEUE_CLASS;
 	}
 
+	/*
+	 * If setscheduling to SCHED_DEADLINE (or changing the parameters
+	 * of a SCHED_DEADLINE task) we need to check if enough bandwidth
+	 * is available. Last of the checks: nothing gives back the
+	 * bandwidth it reserves.
+	 */
+	if ((dl_policy(policy) || dl_task(p)) && sched_dl_overflow(p, policy, attr)) {
+		retval = -EBUSY;
+		goto unlock;
+	}
+
+	p->sched_reset_on_fork = reset_on_fork;
+
 	scx_prepare_setscheduler(p, policy);
 
 	scoped_guard (sched_change, p, queue_flags) {
-- 
2.47.3