linux-kernel - Re: [RFC v2 4/9] sched/fair: Don't use CPU marked as avoid for wakeup and load balance

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Message-ID: <aFyOITguo83GdxKv@yury>
Date: Wed, 25 Jun 2025 20:02:50 -0400
From: Yury Norov <yury.norov@...il.com>
To: Shrikanth Hegde <sshegde@...ux.ibm.com>
Cc: mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
	vincent.guittot@...aro.org, tglx@...utronix.de, maddy@...ux.ibm.com,
	vschneid@...hat.com, dietmar.eggemann@....com, rostedt@...dmis.org,
	kprateek.nayak@....com, huschle@...ux.ibm.com, srikar@...ux.ibm.com,
	linux-kernel@...r.kernel.org, christophe.leroy@...roup.eu,
	linuxppc-dev@...ts.ozlabs.org, gregkh@...uxfoundation.org
Subject: Re: [RFC v2 4/9] sched/fair: Don't use CPU marked as avoid for
 wakeup and load balance

On Thu, Jun 26, 2025 at 12:41:03AM +0530, Shrikanth Hegde wrote:
> Load balancer shouldn't spread CFS tasks into a CPU marked as Avoid. 
> Remove those CPUs from load balancing decisions. 
> 
> At wakeup, don't select a CPU marked as avoid. 
> 
> Signed-off-by: Shrikanth Hegde <sshegde@...ux.ibm.com>
> ---
> while tesing didn't see cpu being marked as avoid while new_cpu is. 
> May need some more probing to see if even cpu can be. if so it could
> lead to crash.  
> 
>  kernel/sched/fair.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 7e2963efe800..406288aef535 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -8546,7 +8546,12 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
>  	}
>  	rcu_read_unlock();
>  
> -	return new_cpu;
> +	/* Don't select a CPU marked as avoid for wakeup */
> +	if (cpu_avoid(new_cpu))
> +		return cpu;
> +	else
> +		return new_cpu;
> +
>  }

There are more 'return's in this function, but you patch only one...

>  
>  /*
> @@ -11662,6 +11667,9 @@ static int sched_balance_rq(int this_cpu, struct rq *this_rq,
>  
>  	cpumask_and(cpus, sched_domain_span(sd), cpu_active_mask);
>  
> +	/* Don't spread load into CPUs marked as avoid */
> +	cpumask_andnot(cpus, cpus, cpu_avoid_mask);
> +
>  	schedstat_inc(sd->lb_count[idle]);
>  
>  redo:
> -- 
> 2.43.0