[PATCH 05/12] sched/deadline: Do not release a departing sleeper's bandwidth twice

[Mathieu Desnoyers]

Commit 41a7968022ca ("sched/deadline: Fix total_bw leak when switching
away from SCHED_DEADLINE while !queued") releases the bandwidth of a
task that leaves SCHED_DEADLINE asleep with no inactive timer pending.
Three cases release what the root domain no longer holds, or never held:

 - the task keeps its parameters. inactive_task_timer() resets them
   once it has released the bandwidth (__dl_clear_params()); left in
   place, task_contending() and task_non_contending() take the task,
   now of another policy, for one with bandwidth when priority
   inheritance boosts it into the deadline class, and a deboost while
   it sleeps releases that bandwidth from total_bw a second time. The
   root domain then admits deadline tasks beyond its capacity;
 - a task deboosted from priority inheritance with no parameters of its
   own, and a special one (SCHED_FLAG_SUGOV), have no bandwidth in
   total_bw. Releasing 0 changes nothing, and the root domain that
   dl_task_root_domain_cpu() finds from the task's mask means nothing
   either: a sleeping task of another policy may have no active CPU in
   its mask, a CPU it is pinned to having gone offline, until its next
   wakeup widens it;
 - migrate_task_rq_dl() clears dl_non_contending when it cannot cancel
   the inactive timer because its handler runs. That handler, once it
   takes the task's locks, finds it out of SCHED_DEADLINE and releases
   total_bw itself.

Release for a task with parameters of its own, not special, and reset
its parameters afterwards, once sub_rq_bw(), which reads its dl_bw, is
done: a handler that runs late then finds no bandwidth to release. A
test of hrtimer_active() would not tell that handler apart: it stays
true after the handler returns, until the hrtimer code clears its
running state, and the bandwidth of a task switched out meanwhile would
never be released.

The deadline_bw_test selftest, added later in this series, reproduces
the first case as pi_former_owner_asleep: in a one-CPU partition with
room, the waiter of its second round is refused as a deadline task,
total_bw having wrapped. The other two were found by reading.

Fixes: 41a7968022ca ("sched/deadline: Fix total_bw leak when switching away from SCHED_DEADLINE while !queued")
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
---
 kernel/sched/deadline.c | 22 +++++++++++++++++++++-
 1 file changed, 21 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index d474fbfd9be6..3c57e5f5fc05 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -3567,6 +3567,8 @@ static void switched_from_dl(struct rq *rq, struct task_struct *p)
 	dec_dl_tasks_cs(p);
 
 	if (!task_on_rq_queued(p)) {
+		bool released = false;
+
 		if (p->dl.dl_non_contending) {
 			/*
 			 * Inactive timer is armed. However, p is leaving
@@ -3579,12 +3581,18 @@ static void switched_from_dl(struct rq *rq, struct task_struct *p)
 			 * release total_bw via __dl_sub().
 			 */
 			sub_running_bw(&p->dl, &rq->dl);
-		} else {
+		} else if (p->dl.dl_runtime && !dl_entity_is_special(&p->dl)) {
 			/*
 			 * No pending inactive timer: the task never ran as
 			 * DEADLINE or the timer already fired while still
 			 * DEADLINE (only dropping running_bw at that time).
 			 * Nobody else will release total_bw, do it now.
+			 *
+			 * Not for a task without parameters of its own, one
+			 * deboosted from priority inheritance, whose mask may
+			 * have no active CPU for dl_task_root_domain_cpu(),
+			 * nor for a special one: neither has bandwidth in
+			 * total_bw.
 			 */
 			unsigned int cpu = dl_task_root_domain_cpu(p);
 			struct dl_bw *dl_b = &cpu_rq(cpu)->rd->dl_bw;
@@ -3592,8 +3600,20 @@ static void switched_from_dl(struct rq *rq, struct task_struct *p)
 			raw_spin_lock(&dl_b->lock);
 			__dl_sub(dl_b, p->dl.dl_bw, dl_bw_cpus(cpu));
 			raw_spin_unlock(&dl_b->lock);
+			released = true;
 		}
 		sub_rq_bw(&p->dl, &rq->dl);
+		/*
+		 * Released: reset the parameters, as inactive_task_timer()
+		 * does. Kept, they would make the task one with bandwidth to
+		 * task_contending() and task_non_contending() once priority
+		 * inheritance boosts it, and a deboost while it sleeps would
+		 * release that bandwidth again. So would the inactive timer's
+		 * handler when it runs late, dl_non_contending cleared under
+		 * it by migrate_task_rq_dl(): it finds !dl_task(p).
+		 */
+		if (released)
+			__dl_clear_params(&p->dl);
 	}
 
 	/*
-- 
2.47.3