linux-kernel - [PATCH] sched/deadline: Fix server stopping with runnable tasks

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Message-ID: <20260113085159.114226-3-gmonaco@redhat.com>
Date: Tue, 13 Jan 2026 09:52:01 +0100
From: Gabriele Monaco <gmonaco@...hat.com>
To: juri.lelli@...hat.com,
	peterz@...radead.org,
	Ingo Molnar <mingo@...hat.com>,
	linux-kernel@...r.kernel.org
Cc: williams@...hat.com,
	Gabriele Monaco <gmonaco@...hat.com>
Subject: [PATCH] sched/deadline: Fix server stopping with runnable tasks

The deadline server can currently stop due to idle although fair tasks
are runnable. This happens essentially when:

* the server is set to idle, a task wakes up, the server stops
* a task wakes up, the server sets itself to idle and stops right away

Address both cases by clearing the server idle flag whenever a fair task
wakes up and accounting also for pending tasks in the definition of idle.

Signed-off-by: Gabriele Monaco <gmonaco@...hat.com>
---

This is really quick and dirty but seems to fix it according to the
models. I also updated the chart to show what is happening here.
I added the same event (server_resume) to the model.

 kernel/sched/deadline.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 138136742871..1070e93350df 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -1376,7 +1376,7 @@ update_stats_dequeue_dl(struct dl_rq *dl_rq, struct sched_dl_entity *dl_se, int
 
 static void update_curr_dl_se(struct rq *rq, struct sched_dl_entity *dl_se, s64 delta_exec)
 {
-	bool idle = rq->curr == rq->idle;
+	bool idle = rq->curr == rq->idle && !rq->nr_running && !rq->ttwu_pending;
 	s64 scaled_delta_exec;
 
 	if (unlikely(delta_exec <= 0)) {
@@ -1560,8 +1560,8 @@ void dl_server_update(struct sched_dl_entity *dl_se, s64 delta_exec)
  * | 8 |       B:zero_laxity-wait       |     |    |
  * |   |                                | <---+    |
  * |   +--------------------------------+          |
- * |     |              ^     ^           2        |
- * |     | 7            | 2   +--------------------+
+ * |     |              ^         ^       2        |
+ * |     | 7            | 2, 1    +----------------+
  * |     v              |
  * |   +-------------+  |
  * +-- | C:idle-wait | -+
@@ -1606,8 +1606,11 @@ void dl_server_update(struct sched_dl_entity *dl_se, s64 delta_exec)
  *   dl_defer_idle = 0
  *
  *
- * [1] A->B, A->D
+ * [1] A->B, A->D, C->B
  * dl_server_start()
+ *   dl_defer_idle = 0;
+ *   if (dl_server_active)
+ *     return; // [B]
  *   dl_server_active = 1;
  *   enqueue_dl_entity()
  *     update_dl_entity(WAKEUP)
@@ -1741,6 +1744,7 @@ void dl_server_start(struct sched_dl_entity *dl_se)
 {
 	struct rq *rq = dl_se->rq;
 
+	dl_se->dl_defer_idle = 0;
 	if (!dl_server(dl_se) || dl_se->dl_server_active)
 		return;
 

base-commit: 0f61b1860cc3f52aef9036d7235ed1f017632193
-- 
2.52.0