[PATCH v5 09/29] sched/deadline: Reset the deadline and the runtime with the parameters

[Mathieu Desnoyers]

A task that leaves SCHED_DEADLINE keeps its parameters until its 0-lag
time, so that leaving and coming back gains nothing. Commit 209a0cbda7a0
("sched/deadline: Improve the tracking of active utilization") moved
their reset to that time. Before it, the replenishment timer did the
reset at the deadline. So a task back after the reset found its
deadline passed, and started a new period. But __dl_clear_params()
resets neither the deadline nor the runtime.

Now a task back after its 0-lag time can be enqueued on its old
deadline and runtime. With runtime left, its 0-lag time comes before
its deadline. Back before that deadline, it would first spend the runtime
of the reservation it gave back. Back after it, a constrained task
would be throttled until its old period ends. "sched/deadline: Do not
lose a throttled task switched out and back" moves the deadline of a
throttled task on. This widens the window before the deadline to about
a period.

A task with no parameters of its own, boosted by a deadline waiter,
keeps the donor's deadline and runtime after its deboost. If it then
enters SCHED_DEADLINE before that deadline, it runs on the donor's
runtime. On v7.4-rc1, a task of 10ms per 100ms ran 40ms of its first
period.

Fix this by resetting the deadline and the runtime in
__dl_clear_params(), and in switched_from_dl() at the deboost of a task
with no parameters. The next enqueue then starts a new period.

The Sashiko AI review found the first case in the second posting of
this series, and the deboost in the third. The deboost_resets_period
case of deadline_bw_test, added later in this series, tests the
deboost.

Fixes: 209a0cbda7a0 ("sched/deadline: Improve the tracking of active utilization")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
---
 kernel/sched/deadline.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index cf24d2df6bfc..cc43c5eeae51 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -3573,6 +3573,16 @@ static void switched_from_dl(struct rq *rq, struct task_struct *p)
 	if (task_on_rq_queued(p) && p->dl.dl_runtime)
 		task_non_contending(&p->dl, false);
 
+	/*
+	 * A task with no parameters of its own was boosted into the class.
+	 * Its deadline and runtime are the donor's. Reset them, so that its
+	 * next entry into the class starts a new period.
+	 */
+	if (!p->dl.dl_runtime) {
+		p->dl.deadline = 0;
+		p->dl.runtime = 0;
+	}
+
 	/*
 	 * In case a task is setscheduled out from SCHED_DEADLINE we need to
 	 * keep track of that on its cpuset (for correct bandwidth tracking).
@@ -3999,6 +4009,9 @@ static void __dl_clear_params(struct sched_dl_entity *dl_se)
 	dl_se->dl_bw			= 0;
 	dl_se->dl_density		= 0;
 
+	dl_se->deadline			= 0;
+	dl_se->runtime			= 0;
+
 	dl_se->dl_throttled		= 0;
 	dl_se->dl_yielded		= 0;
 	dl_se->dl_non_contending	= 0;
-- 
2.47.3