[PATCH v2 1/1] sched/deadline: Fix dl_server runtime calculation formula

[Kuyo Chang <kuyo.chang@mediatek.com>]

From: kuyo chang <kuyo.chang@mediatek.com>

[Symptom]
The calculation formula for dl_server runtime is based on
Frequency/capacity scale-invariance.
This will cause excessive RT latency (expect absolute time).

[Analysis]
Consider the following case under a Big.LITTLE architecture:

Assume the runtime is: 50,000,000 ns, and Frequency/capacity
scale-invariance defined as below:

Frequency scale-invariance: 100
Capacity scale-invariance: 50
First by Frequency scale-invariance,
the runtime is scaled to 50,000,000 * 100 >> 10 = 4,882,812
Then by capacity scale-invariance,
it is further scaled to 4,882,812 * 50 >> 10 = 238,418.

So it will scaled to 238,418 ns.

[Solution]
The runtime for dl_server should be fixed time
asis RT bandwidth control.
Fix the runtime calculation formula for the dl_server.

Signed-off-by: kuyo chang <kuyo.chang@mediatek.com>
Acked-by: Juri Lelli <juri.lelli@redhat.com>
Suggested-by: Peter Zijlstra <peterz@infradead.org>

v1: https://lore.kernel.org/all/20250614020524.631521-1-kuyo.chang@mediatek.com/

---
 kernel/sched/deadline.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index ad45a8fea245..f68a158d01e9 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -1504,7 +1504,9 @@ static void update_curr_dl_se(struct rq *rq, struct sched_dl_entity *dl_se, s64
 	if (dl_entity_is_special(dl_se))
 		return;
 
-	scaled_delta_exec = dl_scaled_delta_exec(rq, dl_se, delta_exec);
+	scaled_delta_exec = delta_exec;
+	if (!dl_se->dl_server)
+		scaled_delta_exec = dl_scaled_delta_exec(rq, dl_se, delta_exec);
 
 	dl_se->runtime -= scaled_delta_exec;
 
@@ -1624,7 +1626,9 @@ void dl_server_update_idle_time(struct rq *rq, struct task_struct *p)
 	if (delta_exec < 0)
 		return;
 
-	scaled_delta_exec = dl_scaled_delta_exec(rq, &rq->fair_server, delta_exec);
+	scaled_delta_exec = delta_exec;
+	if (!rq->fair_server.dl_server)
+		scaled_delta_exec = dl_scaled_delta_exec(rq, &rq->fair_server, delta_exec);
 
 	rq->fair_server.runtime -= scaled_delta_exec;
 
-- 
2.45.2