linux-kernel - [PATCH v7 1/1] sched: update the rq->avg_idle when a task is moved to an idle CPU

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Message-ID: <20251226063203.5947-2-shijie8@gmail.com>
Date: Fri, 26 Dec 2025 14:32:03 +0800
From: Huang Shijie <shijie8@...il.com>
To: mingo@...hat.com,
	peterz@...radead.org,
	vincent.guittot@...aro.org
Cc: dietmar.eggemann@....com,
	rostedt@...dmis.org,
	bsegall@...gle.com,
	mgorman@...e.de,
	vschneid@...hat.com,
	linux-kernel@...r.kernel.org,
	vineethr@...ux.ibm.com,
	kprateek.nayak@....com,
	cl@...ux.com,
	Huang Shijie <shijie8@...il.com>
Subject: [PATCH v7 1/1] sched: update the rq->avg_idle when a task is moved to an idle CPU

In the newidle balance, the rq->idle_stamp may set to a non-zero value
if it cannot pull any task.

In the wakeup, it will detect the rq->idle_stamp, and updates
the rq->avg_idle, then ends the CPU idle status by setting rq->idle_stamp
to zero.

Besides the wakeup, current code does not end the CPU idle status
when a task is moved to the idle CPU, such as fork/clone, execve,
or other cases. In order to get more accurate rq->avg_idle,
we need to update it at more places(not only the wakeup).

This patch introduces a helper: update_rq_avg_idle().
And uses it in put_prev_task_idle(), so it will update the rq->avg_idle
when a task is moved to an idle CPU at:
   -- wakeup
   -- fork/clone
   -- execve
   -- idle balance
   -- other cases

Signed-off-by: Huang Shijie <shijie8@...il.com>
---
 kernel/sched/core.c  | 27 +++++++++++++++------------
 kernel/sched/idle.c  |  1 +
 kernel/sched/sched.h |  1 +
 3 files changed, 17 insertions(+), 12 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 5b17d8e3cb55..ad52f3a3c6bf 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3609,6 +3609,21 @@ static inline void ttwu_do_wakeup(struct task_struct *p)
 	trace_sched_wakeup(p);
 }
 
+void update_rq_avg_idle(struct rq *rq)
+{
+	if (rq->idle_stamp) {
+		u64 delta = rq_clock(rq) - rq->idle_stamp;
+		u64 max = 2*rq->max_idle_balance_cost;
+
+		update_avg(&rq->avg_idle, delta);
+
+		if (rq->avg_idle > max)
+			rq->avg_idle = max;
+
+		rq->idle_stamp = 0;
+	}
+}
+
 static void
 ttwu_do_activate(struct rq *rq, struct task_struct *p, int wake_flags,
 		 struct rq_flags *rf)
@@ -3644,18 +3659,6 @@ ttwu_do_activate(struct rq *rq, struct task_struct *p, int wake_flags,
 		p->sched_class->task_woken(rq, p);
 		rq_repin_lock(rq, rf);
 	}
-
-	if (rq->idle_stamp) {
-		u64 delta = rq_clock(rq) - rq->idle_stamp;
-		u64 max = 2*rq->max_idle_balance_cost;
-
-		update_avg(&rq->avg_idle, delta);
-
-		if (rq->avg_idle > max)
-			rq->avg_idle = max;
-
-		rq->idle_stamp = 0;
-	}
 }
 
 /*
diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
index 65eb8f8c1a5d..aba5ad53c07d 100644
--- a/kernel/sched/idle.c
+++ b/kernel/sched/idle.c
@@ -460,6 +460,7 @@ static void put_prev_task_idle(struct rq *rq, struct task_struct *prev, struct t
 {
 	update_curr_idle(rq);
 	scx_update_idle(rq, false, true);
+	update_rq_avg_idle(rq);
 }
 
 static void set_next_task_idle(struct rq *rq, struct task_struct *next, bool first)
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 3ceaa9dc9a9e..6e3dd8c975e0 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1651,6 +1651,7 @@ static inline struct cfs_rq *group_cfs_rq(struct sched_entity *grp)
 
 #endif /* !CONFIG_FAIR_GROUP_SCHED */
 
+extern void update_rq_avg_idle(struct rq *rq);
 extern void update_rq_clock(struct rq *rq);
 
 /*
-- 
2.43.0