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

[Shrikanth Hegde <sshegde@linux.ibm.com>]

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?


While thinking about this, are there any such system which has vCPUs and
overcommits and still has energy model backing it?

Highly unlikely. So, I am planning to put a warning there and see if any
such usage exists there.

> 
>>   			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?
> 

We have this pattern in select_task_rq_fair

cpu = smp_processor_id();
new_cpu = prev_cpu;

task is waking up after a while, so likely prev_cpu is marked as paravirt
and in such cases we should return current cpu. if current cpu is paravirt(unlikely),
and prev_cpu is also paravirt, then should return current cpu.
In next sched tick it will be pushed out.

select_idle_sibling(p, prev_cpu, new_cpu); - (new_cpu will remain prev_cpu if wake_affine doesn't change it)
Will have to change the prototype to send current cpu as well.