linux-kernel - Re: [PATCH v4 1/2] sched/fair: set rq->idle_stamp at the end of the sched_balance

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Message-ID: <1b0facef-8d3c-45fa-a490-506204e1395d@linux.ibm.com>
Date: Fri, 28 Nov 2025 15:37:44 +0530
From: Madadi Vineeth Reddy <vineethr@...ux.ibm.com>
To: Huang Shijie <shijie@...amperecomputing.com>
Cc: mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, 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,
        kprateek.nayak@....com, Madadi Vineeth Reddy <vineethr@...ux.ibm.com>
Subject: Re: [PATCH v4 1/2] sched/fair: set rq->idle_stamp at the end of the
 sched_balance_newidle

Hi Huang,

On 28/11/25 13:24, Huang Shijie wrote:
> Save the idle_stamp at the beginning of sched_balance_newidle(),
> if it cannot pull any task, set it for rq->idle_stamp.
> 
> This patch does not change the logic of rq->idle_stamp.
> 
> Signed-off-by: Huang Shijie <shijie@...amperecomputing.com>
> ---
>  kernel/sched/fair.c | 12 ++++++++----
>  1 file changed, 8 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 769d7b7990df..c1a8fa043156 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -12862,6 +12862,7 @@ static int sched_balance_newidle(struct rq *this_rq, struct rq_flags *rf)
>  	u64 t0, t1, curr_cost = 0;
>  	struct sched_domain *sd;
>  	int pulled_task = 0;
> +	u64 idle_stamp;
>  
>  	update_misfit_status(NULL, this_rq);
>  
> @@ -12877,7 +12878,9 @@ static int sched_balance_newidle(struct rq *this_rq, struct rq_flags *rf)
>  	 * for CPU_NEWLY_IDLE, such that we measure the this duration
>  	 * as idle time.
>  	 */
> -	this_rq->idle_stamp = rq_clock(this_rq);
> +	idle_stamp = rq_clock(this_rq);
> +
> +	this_rq->idle_stamp = 0;

IIUC, by setting this_rq->idle_stamp = 0 at the beginning, any call to update_rq_avg_idle() during
load balancing when tasks are pulled will fail the if (rq->idle_stamp) check, preventing the average
idle time from being updated.

Thanks,
Vineeth

>  
>  	/*
>  	 * Do not pull tasks towards !active CPUs...
> @@ -12989,10 +12992,11 @@ static int sched_balance_newidle(struct rq *this_rq, struct rq_flags *rf)
>  	if (time_after(this_rq->next_balance, next_balance))
>  		this_rq->next_balance = next_balance;
>  
> -	if (pulled_task)
> -		this_rq->idle_stamp = 0;
> -	else
> +	if (!pulled_task) {
> +		/* Set it here on purpose. */
> +		this_rq->idle_stamp = idle_stamp;
>  		nohz_newidle_balance(this_rq);
> +	}
>  
>  	rq_repin_lock(this_rq, rf);
>