linux-kernel - [PATCH v2 2/3] sched: update the rq->avg_idle when a task is moved to an idle CPU

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Message-Id: <20251127050353.1089724-3-shijie@os.amperecomputing.com>
Date: Thu, 27 Nov 2025 13:03:52 +0800
From: Huang Shijie <shijie@...amperecomputing.com>
To: mingo@...hat.com,
	peterz@...radead.org,
	juri.lelli@...hat.com,
	vincent.guittot@...aro.org
Cc: patches@...erecomputing.com,
	cl@...ux.com,
	Shubhang@...amperecomputing.com,
	dietmar.eggemann@....com,
	rostedt@...dmis.org,
	bsegall@...gle.com,
	mgorman@...e.de,
	linux-kernel@...r.kernel.org,
	vschneid@...hat.com,
	vineethr@...ux.ibm.com,
	Huang Shijie <shijie@...amperecomputing.com>
Subject: [PATCH v2 2/3] 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.

This patch introduces a helper: update_rq_avg_idle().
And uses it in activate_task(), 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 <shijie@...amperecomputing.com>
---
 kernel/sched/core.c  | 28 ++++++++++++++++------------
 kernel/sched/sched.h |  2 ++
 2 files changed, 18 insertions(+), 12 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 0c4ff93eeb78..a946f3604548 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2135,6 +2135,7 @@ void activate_task(struct rq *rq, struct task_struct *p, int flags)
 		sched_mm_cid_migrate_to(rq, p);
 
 	enqueue_task(rq, p, flags);
+	update_rq_avg_idle(rq);
 
 	WRITE_ONCE(p->on_rq, TASK_ON_RQ_QUEUED);
 	ASSERT_EXCLUSIVE_WRITER(p->on_rq);
@@ -2412,6 +2413,21 @@ static inline bool is_cpu_allowed(struct task_struct *p, int cpu)
 	return cpu_online(cpu);
 }
 
+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;
+	}
+}
+
 /*
  * This is how migration works:
  *
@@ -3645,18 +3661,6 @@ 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/sched.h b/kernel/sched/sched.h
index b419a4d98461..0e8aef1cef96 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -242,6 +242,8 @@ static inline void update_avg(u64 *avg, u64 sample)
 	*avg += diff / 8;
 }
 
+extern void update_rq_avg_idle(struct rq *);
+
 /*
  * Shifting a value by an exponent greater *or equal* to the size of said value
  * is UB; cap at size-1.
-- 
2.40.1