linux-kernel - Re: [RFC PATCH v3 05/10] sched/fair: Don't consider paravirt CPUs for wakeup and load balance

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Message-ID: <d0aa1a34-2097-405c-b431-907fd973ad96@amd.com>
Date: Thu, 11 Sep 2025 10:53:45 +0530
From: K Prateek Nayak <kprateek.nayak@....com>
To: Shrikanth Hegde <sshegde@...ux.ibm.com>, <mingo@...hat.com>,
	<peterz@...radead.org>, <juri.lelli@...hat.com>,
	<vincent.guittot@...aro.org>, <tglx@...utronix.de>, <yury.norov@...il.com>,
	<maddy@...ux.ibm.com>, <linux-kernel@...r.kernel.org>,
	<linuxppc-dev@...ts.ozlabs.org>, <gregkh@...uxfoundation.org>
CC: <vschneid@...hat.com>, <iii@...ux.ibm.com>, <huschle@...ux.ibm.com>,
	<rostedt@...dmis.org>, <dietmar.eggemann@....com>, <vineeth@...byteword.org>,
	<jgross@...e.com>, <pbonzini@...hat.com>, <seanjc@...gle.com>
Subject: Re: [RFC PATCH v3 05/10] sched/fair: Don't consider paravirt CPUs for
 wakeup and load balance

Hello Shrikanth,

On 9/10/2025 11:12 PM, Shrikanth Hegde wrote:
> @@ -8563,7 +8563,7 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
>  		if (!is_rd_overutilized(this_rq()->rd)) {
>  			new_cpu = find_energy_efficient_cpu(p, prev_cpu);
>  			if (new_cpu >= 0)
> -				return new_cpu;
> +				goto check_new_cpu;

Should this fallback to the overutilized path if the most energy
efficient CPU is found to be paravirtualized or should
find_energy_efficient_cpu() be made aware of it?

>  			new_cpu = prev_cpu;
>  		}
>  
> @@ -8605,7 +8605,12 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
>  	}
>  	rcu_read_unlock();
>  
> -	return new_cpu;
> +	/* If newly found or prev_cpu is a paravirt cpu, use current cpu */
> +check_new_cpu:
> +	if (is_cpu_paravirt(new_cpu))
> +		return cpu;
> +	else

nit. redundant else.

> +		return new_cpu;
>  }
>  
>  /*
> @@ -11734,6 +11739,12 @@ static int sched_balance_rq(int this_cpu, struct rq *this_rq,
>  
>  	cpumask_and(cpus, sched_domain_span(sd), cpu_active_mask);
>  
> +#ifdef CONFIG_PARAVIRT
> +	/* Don't spread load to paravirt CPUs */
> +	if (static_branch_unlikely(&cpu_paravirt_push_tasks))
> +		cpumask_andnot(cpus, cpus, cpu_paravirt_mask);
> +#endif

Can something similar be also be done in select_idle_sibling() and
sched_balance_find_dst_cpu() for wakeup path?

> +
>  	schedstat_inc(sd->lb_count[idle]);
>  
>  redo:
-- 
Thanks and Regards,
Prateek