Re: [PATCH v4 2/9] sched/topology: Extract "imb_numa_nr" calculation into a separate helper

[K Prateek Nayak]

Hello Dietmar,

On 3/16/2026 1:54 PM, Dietmar Eggemann wrote:
> Hi Prateek,
> 
> On 16.03.26 04:41, K Prateek Nayak wrote:
>> Hello Dietmar,
>>
>> On 3/16/2026 5:48 AM, Dietmar Eggemann wrote:
> 
> [...]
> 
>> Indeed! "imb_numa_nr" only makes sense when looking at NUMA domains
>> and having it assigned to 1 for lower domains is harmless
>> (but wasteful indeed). I'm 99% sure we can simply do:
>>
>>   (Only build tested)
>>
>> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
>> index 43150591914b..e9068a809dbc 100644
>> --- a/kernel/sched/topology.c
>> +++ b/kernel/sched/topology.c
>> @@ -2623,9 +2623,6 @@ static void adjust_numa_imbalance(struct sched_domain *sd_llc)
>>  	else
>>  		imb = nr_llcs;
>>  
>> -	imb = max(1U, imb);
>> -	sd_llc->parent->imb_numa_nr = imb;
>> -
>>  	/*
>>  	 * Set span based on the first NUMA domain.
>>  	 *
>> @@ -2639,10 +2636,14 @@ static void adjust_numa_imbalance(struct sched_domain *sd_llc)
>>  	while (parent && !(parent->flags & SD_NUMA))
>>  		parent = parent->parent;
>>  
>> -	imb_span = parent ? parent->span_weight : sd_llc->parent->span_weight;
>> +	/* No NUMA domain to adjust imbalance for! */
>> +	if (!parent)
>> +		return;
>> +
>> +	imb = max(1U, imb);
>> +	imb_span = parent->span_weight;
>>  
>>  	/* Update the upper remainder of the topology */
>> -	parent = sd_llc->parent;
>>  	while (parent) {
>>  		int factor = max(1U, (parent->span_weight / imb_span));
>>  
>> ---
>>
>> If we have NUMA domains, we definitely have NODE and NODE sets neither
>> SD_SHARE_LLC, nor SD_NUMA so likely sd->parent is PKG / NODE domain and
>> NUMA has to start at sd->parent->parent and it has to break at the first
>> SD_NUMA domains.
>>
>> If it doesn't exist, we don't have any NUMA domains and nothing to worry
>> about, and if we do, the final loop will adjust the NUMA imbalance.
>>
>> Thoughts? Again, this commit was kept 1:1 with the previous loop but we
>> can always improve :-)
> Ah, I see!
> 
> This would work, IMHO.
> 
> Tested on qemu-system-aarch64 w/
> 
>   -smp 8,sockets=2,clusters=2,cores=2,threads=1
> 
> Are you aware of a setup in which PKG would survive between MC and
> lowest NUMA?

On x86, you can have:

  -smp 8,sockets=2,dies=2,cores=2,threads=1

and each "die" will appear as an MC within the socket so we get

  NUMA {         0-7         }
  NODE {   0-3   } {   4-7   }
  PKG  {   0-3   } {   4-7   }
  MC   {0,1} {2,3} {4,5} {6,7}

In the above case, NODE is degenerated since it matches with PKG
and MC, PKG, NUMA survive at the end.

-- 
Thanks and Regards,
Prateek