linux-kernel - Re: [PATCH 2/4] sched/fair: Small cleanup to sched_balance

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Message-ID: <01dcb63e-9ebd-42ca-9418-a822bf081bfc@linux.ibm.com>
Date: Wed, 12 Nov 2025 20:58:23 +0530
From: Shrikanth Hegde <sshegde@...ux.ibm.com>
To: Peter Zijlstra <peterz@...radead.org>, vincent.guittot@...aro.org
Cc: mingo@...hat.com, juri.lelli@...hat.com, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        vschneid@...hat.com, linux-kernel@...r.kernel.org,
        Chris Mason
 <clm@...a.com>,
        Joseph Salisbury <joseph.salisbury@...cle.com>,
        Adam Li <adamli@...amperecomputing.com>,
        Hazem Mohamed Abuelfotoh <abuehaze@...zon.com>,
        Josh Don <joshdon@...gle.com>
Subject: Re: [PATCH 2/4] sched/fair: Small cleanup to sched_balance_newidle()



On 11/12/25 8:38 PM, Peter Zijlstra wrote:
> On Wed, Nov 12, 2025 at 03:42:41PM +0100, Peter Zijlstra wrote:
> 
>>> if sd is null, i think we are skipping these compared to earlier.
>>>
>>>          t0 = sched_clock_cpu(this_cpu);
>>>          sched_balance_update_blocked_averages(this_cpu);
>>
>> let me pull that sched_balance_update_blocked_averages() thing up a few
>> lines.
> 
> Something like so..
> 
> ---
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -9946,15 +9946,11 @@ static unsigned long task_h_load(struct
>   }
>   #endif /* !CONFIG_FAIR_GROUP_SCHED */
>   
> -static void sched_balance_update_blocked_averages(int cpu)
> +static void __sched_balance_update_blocked_averages(struct rq *rq)
>   {
>   	bool decayed = false, done = true;
> -	struct rq *rq = cpu_rq(cpu);
> -	struct rq_flags rf;
>   
> -	rq_lock_irqsave(rq, &rf);
>   	update_blocked_load_tick(rq);
> -	update_rq_clock(rq);
>   
>   	decayed |= __update_blocked_others(rq, &done);
>   	decayed |= __update_blocked_fair(rq, &done);
> @@ -9962,7 +9958,15 @@ static void sched_balance_update_blocked
>   	update_blocked_load_status(rq, !done);
>   	if (decayed)
>   		cpufreq_update_util(rq, 0);
> -	rq_unlock_irqrestore(rq, &rf);
> +}
> +
> +static void sched_balance_update_blocked_averages(int cpu)
> +{
> +	struct rq *rq = cpu_rq(cpu);
> +
> +	guard(rq_lock_irqsave)(rq);
> +	update_rq_clock(rq);
> +	__sched_balance_update_blocked_averages(rq);
>   }
>   
>   /********** Helpers for sched_balance_find_src_group ************************/
> @@ -12865,6 +12869,8 @@ static int sched_balance_newidle(struct
>   	if (!cpu_active(this_cpu))
>   		return 0;
>   
> +	__sched_balance_update_blocked_averages(this_rq);
> +

is this done only when sd == null ?

>   	/*
>   	 * This is OK, because current is on_cpu, which avoids it being picked
>   	 * for load-balance and preemption/IRQs are still disabled avoiding
> @@ -12891,7 +12897,6 @@ static int sched_balance_newidle(struct
>   	raw_spin_rq_unlock(this_rq);
>   
>   	t0 = sched_clock_cpu(this_cpu);
> -	sched_balance_update_blocked_averages(this_cpu);
>   
>   	rcu_read_lock();
>   	for_each_domain(this_cpu, sd) {

Referring to commit,
9d783c8dd112a (sched/fair: Skip update_blocked_averages if we are defering load balance)
I think vincent added the max_newidle_lb_cost check since sched_balance_update_blocked_averages is costly.