linux-kernel - Re: [PATCH v6 2/2] sched: update the rq->avg_idle when a task is moved to an idle CPU

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Message-ID: <99c18623-4533-4dde-a765-ea43c85ee02f@amperemail.onmicrosoft.com>
Date: Mon, 15 Dec 2025 17:35:45 +0800
From: Shijie Huang <shijie@...eremail.onmicrosoft.com>
To: Dietmar Eggemann <dietmar.eggemann@....com>,
 Huang Shijie <shijie@...amperecomputing.com>, mingo@...hat.com,
 peterz@...radead.org, juri.lelli@...hat.com, vincent.guittot@...aro.org
Cc: patches@...erecomputing.com, cl@...ux.com,
 Shubhang@...amperecomputing.com, rostedt@...dmis.org, bsegall@...gle.com,
 mgorman@...e.de, linux-kernel@...r.kernel.org, vschneid@...hat.com,
 vineethr@...ux.ibm.com, kprateek.nayak@....com
Subject: Re: [PATCH v6 2/2] sched: update the rq->avg_idle when a task is
 moved to an idle CPU


On 12/12/2025 22:22, Dietmar Eggemann wrote:
>>> So you could move (1) back to (2) avoiding the 'if rq->idle_stamp' for
>>> the sched_change pattern for instance?
>> Could you please tell me what is "avoiding the 'if rq->idle_stamp' for
>> the sched_change pattern" ?
>>
>> Sorry, I do not understand your meaning.
> sched_change uses dequeue_task()/enqueue_task() for a queued task to
> change prio, policy, sched params, taskgroups, etc.

For sched_change, the dequeue_task()/enqueue_task() only work when

the queued task has TASK_ON_RQ_QUEUED flags. The TASK_ON_RQ_QUEUED

is set in activate_task().


   1.) For this active task, if the sched_change makes it 
dequeue_task()/enqueue_task() on

       current CPU, it's okay. Since current CPU is not in the newidle, 
the "rq->idle_stamp" is 0 at this case.

       This patch works fine.


  2.) For this active task, if the sched_change makes it 
dequeue_task()/enqueue_task() on an another CPU,

       it's okay too.

          2.1) If the another CPU's idle_stamp is 0, the another CPU is 
busy now.

                  The sched_change works fine with this patch.

          2.2) If the another CPU's idle_stamp is not 0, the 
sched_change also works fine with this patch.

                 Since the sched_change is breaking the idle state of 
the another CPU by moving an active

                 task to an idle CPU. It makes sense.


Thanks

Huang Shijie