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: <e3826a8d-98cc-4497-85ce-5d92e0556cea@linux.ibm.com>
Date: Thu, 11 Sep 2025 21:26:56 +0530
From: Shrikanth Hegde <sshegde@...ux.ibm.com>
To: K Prateek Nayak <kprateek.nayak@....com>
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,
        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
Subject: Re: [RFC PATCH v3 05/10] sched/fair: Don't consider paravirt CPUs for
 wakeup and load balance



On 9/11/25 10:53 AM, K Prateek Nayak wrote:
> 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.
> 

Do you mean "is_cpu_paravirt(new_cpu) ? cpu; new_cpu"

This needs to return cpu instead of true/false. maybe i not seeing the obvious.

>> +		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?

That's a good suggestion. don't make a choice which is a paravirt CPU.
Will explore.

> 
>> +
>>   	schedstat_inc(sd->lb_count[idle]);
>>   
>>   redo: