linux-kernel - [PATCH 1/1] sched/deadline: Fix fair_server runtime calculation formula

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Message-ID: <20250614020524.631521-1-kuyo.chang@mediatek.com>
Date: Sat, 14 Jun 2025 10:04:55 +0800
From: Kuyo Chang <kuyo.chang@...iatek.com>
To: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
	Juri Lelli <juri.lelli@...hat.com>, Vincent Guittot
	<vincent.guittot@...aro.org>, Dietmar Eggemann <dietmar.eggemann@....com>,
	Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>, Mel
 Gorman <mgorman@...e.de>, Valentin Schneider <vschneid@...hat.com>, Matthias
 Brugger <matthias.bgg@...il.com>, AngeloGioacchino Del Regno
	<angelogioacchino.delregno@...labora.com>
CC: jstultz <jstultz@...gle.com>, kuyo chang <kuyo.chang@...iatek.com>,
	<linux-kernel@...r.kernel.org>, <linux-arm-kernel@...ts.infradead.org>,
	<linux-mediatek@...ts.infradead.org>
Subject: [PATCH 1/1] sched/deadline: Fix fair_server runtime calculation formula

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

[Symptom]
The calculation formula for fair_server runtime is based on
Frequency/CPU 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 FIE/CIE as below
FIE: 100
CIE:50
First by FIE, the runtime is scaled to 50,000,000 * 100 >> 10 = 4,882,812
Then by CIE, it is further scaled to 4,882,812 * 50 >> 10 = 238,418.

So it will scaled to 238,418 ns.

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

Signed-off-by: kuyo chang <kuyo.chang@...iatek.com>
---
 kernel/sched/deadline.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index ad45a8fea245..8bfa846cf0dc 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -1504,7 +1504,10 @@ 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);
+	if (dl_se == &rq->fair_server)
+		scaled_delta_exec = delta_exec;
+	else
+		scaled_delta_exec = dl_scaled_delta_exec(rq, dl_se, delta_exec);
 
 	dl_se->runtime -= scaled_delta_exec;
 
@@ -1611,7 +1614,7 @@ static void update_curr_dl_se(struct rq *rq, struct sched_dl_entity *dl_se, s64
  */
 void dl_server_update_idle_time(struct rq *rq, struct task_struct *p)
 {
-	s64 delta_exec, scaled_delta_exec;
+	s64 delta_exec;
 
 	if (!rq->fair_server.dl_defer)
 		return;
@@ -1624,9 +1627,7 @@ 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);
-
-	rq->fair_server.runtime -= scaled_delta_exec;
+	rq->fair_server.runtime -= delta_exec;
 
 	if (rq->fair_server.runtime < 0) {
 		rq->fair_server.dl_defer_running = 0;
-- 
2.45.2