[PATCH v5 02/29] sched: Refuse SCHED_FLAG_KEEP_PARAMS that would change the policy or the class

[Mathieu Desnoyers]

A sched_setattr() request with SCHED_FLAG_KEEP_PARAMS can ask for
another policy. __sched_setscheduler() then applies only half of the
change, and returns 0:
- SCHED_OTHER to SCHED_BATCH leaves the policy as it was.
- SCHED_DEADLINE to SCHED_OTHER sets DEQUEUE_CLASS. switched_from_dl()
  then runs on a task that stays in the class. It would release
  bandwidth the task still uses.
- SCHED_DEADLINE on a task of another policy reserves bandwidth in
  sched_dl_overflow(). The task keeps its policy and a dl_bw of 0, so
  nothing releases that bandwidth. For a task boosted into the deadline
  class, it is not even a class change.

SCHED_FLAG_KEEP_PARAMS came with utilization clamping. With
SCHED_FLAG_KEEP_POLICY, it lets sched_setattr() change only the clamps
of a task. So __sched_setscheduler() skips __setscheduler_params(),
which sets p->policy, and the sched_class assignment. Nothing checks
that the request keeps the policy.

In testing on v7.4-rc1, a deadline waiter boosted a SCHED_OTHER mutex
owner with a 40ms slice. One such call on the owner left 0.4 CPU in
total_bw, until the next rebuild of the root domains.

Fix this by refusing a request for another policy or class with
EINVAL. Move sched_dl_overflow() after it, as the last check. Nothing
gives back the bandwidth it reserves. A refused request also leaves
sched_reset_on_fork alone.

With this patch, that call is refused and total_bw does not change. The
params_keep_boosted case of deadline_bw_test, added later in this
series, tests it.

Juri Lelli found the class change, from a report by Tejun Heo. The
Sashiko AI review of the first posting of this series found the policy
change and the boosted case.

Link: https://lore.kernel.org/all/2b5fb3f22bca6aa383be8f4781923392@xxxxxxxxxx/
Link: https://lore.kernel.org/all/20261103-b4-sched-dl-bw-fixes-v1-3-dd761fe005b4@xxxxxxxxxx/
Fixes: a509a7cd7974 ("sched/uclamp: Extend sched_setattr() to support utilization clamping")
Reviewed-by: Juri Lelli <juri.lelli@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
---
 kernel/sched/syscalls.c | 40 +++++++++++++++++++++++++++++-----------
 1 file changed, 29 insertions(+), 11 deletions(-)

diff --git a/kernel/sched/syscalls.c b/kernel/sched/syscalls.c
index 2af550f92ac6..0e8b5c7324f6 100644
--- a/kernel/sched/syscalls.c
+++ b/kernel/sched/syscalls.c
@@ -647,17 +647,6 @@ 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);
@@ -677,9 +666,38 @@ int __sched_setscheduler(struct task_struct *p,
 	prev_class = p->sched_class;
 	next_class = __setscheduler_class(policy, newprio);
 
+	/*
+	 * With SCHED_FLAG_KEEP_PARAMS, the change below skips
+	 * __setscheduler_params() and the sched_class assignment. The
+	 * policy and the class stay as they are. Refuse a request for
+	 * another policy or class, rather than carry out half of it.
+	 *
+	 * The class depends on the priority, which priority inheritance
+	 * can raise. It also depends on whether a BPF scheduler is in
+	 * charge. So test the class too, not only the policy.
+	 */
+	if ((attr->sched_flags & SCHED_FLAG_KEEP_PARAMS) &&
+	    (policy != p->policy || prev_class != next_class)) {
+		retval = -EINVAL;
+		goto unlock;
+	}
+
 	if (prev_class != next_class)
 		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. Keep this check last. If a later check failed,
+	 * nothing would give 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