Re: [PATCH RFC] sched/fair: bump sd->max_newidle_lb_cost when newidle balance fails

[Chris Mason <clm@meta.com>]

On 6/26/25 3:00 AM, Peter Zijlstra wrote:
> On Tue, Jun 24, 2025 at 01:48:08PM -0700, Chris Mason wrote:

[ ... ]

> For the non-RFC version, please split this into a code move and a code
> change -- I had to stare waaay to long to spot the difference (if we
> keep this code movement at all).

Sure

> 
>>  /*
>>   * Check this_cpu to ensure it is balanced within domain. Attempt to move
>>   * tasks if there is an imbalance.
>> @@ -11782,12 +11808,14 @@ static int sched_balance_rq(int this_cpu, struct rq *this_rq,
>>  
>>  	group = sched_balance_find_src_group(&env);
>>  	if (!group) {
>> +		update_newidle_cost(sd, sd->max_newidle_lb_cost + sd->max_newidle_lb_cost / 2);
>>  		schedstat_inc(sd->lb_nobusyg[idle]);
>>  		goto out_balanced;
>>  	}
>>  
>>  	busiest = sched_balance_find_src_rq(&env, group);
>>  	if (!busiest) {
>> +		update_newidle_cost(sd, sd->max_newidle_lb_cost + sd->max_newidle_lb_cost / 2);
>>  		schedstat_inc(sd->lb_nobusyq[idle]);
>>  		goto out_balanced;
>>  	}
> 
> So sched_balance_rq() is used for pretty much all load-balancing, not
> just newidle.
> 
> Either make this conditional like:
> 
> 	if (idle == CPU_NEWLY_IDLE)
> 		update_newidle_cost(...);
> 
> or do it all the callsite, where we find !pulled_task (ie failure).
> 
> Specifically, we already do update_newidle_cost() there, perhaps inflate
> the cost there instead?
> 
> 	if (!pulled_tasks)
> 		domain_cost += sysctl_sched_migration_cost;

Got it, I'll play with that.  Vincent, was there a benchmark I can use
to see if I've regressed the case you were focused on?

-chris