Re: [PATCH v3 3/8] sched/topology: Switch to assigning "sd->shared" from s_data

["Chen, Yu C" <yu.c.chen@intel.com>]

On 2/6/2026 1:20 PM, K Prateek Nayak wrote:
> Hello Valentin,
> 
> On 2/5/2026 10:23 PM, Valentin Schneider wrote:
>> On 20/01/26 11:32, K Prateek Nayak wrote:

[...]

>>
>> We now have two if's looking for the highest_flag_domain(i, SD_SHARE_LLC),
>> but given this needs to write the sd->imb_numa_nr for every SD I couldn't
>> factorize this into something that looked sane :(
> 
> Yeah! The "imb_numa_nr" cares about the "sd_llc" *after* we've crossed
> it and "sd->shared" assignment cares when we are *at* the sd_llc.
> 
> Since we have to assign the "sd->shared" before claim_allocations(),
> I couldn't find a better spot to assign it.
> 
> That said, "imb_numa_nr" calculation can be modified to use the "sd_llc"
> and its "parent". I'll let you be the judge of whether the following is
> better or worse ;-)
> 
>    (Only build tested)
> 
> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> index ac268da91778..e98bb812de35 100644
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c
> @@ -2614,13 +2614,23 @@ build_sched_domains(const struct cpumask *cpu_map, struct sched_domain_attr *att
>   		unsigned int imb_span = 1;
>   
>   		for (sd = *per_cpu_ptr(d.sd, i); sd; sd = sd->parent) {
> -			struct sched_domain *child = sd->child;
> +			struct sched_domain *parent = sd->parent;
>   
> -			if (!(sd->flags & SD_SHARE_LLC) && child &&
> -			    (child->flags & SD_SHARE_LLC)) {
> -				struct sched_domain __rcu *top_p;
> +			/* Topmost SD_SHARE_LLC domain. */
> +			if ((sd->flags & SD_SHARE_LLC) &&
> +			    (!parent || !(parent->flags & SD_SHARE_LLC))) {
> +				int sd_id = cpumask_first(sched_domain_span(sd));
> +				struct sched_domain *top_p;
>   				unsigned int nr_llcs;
>   
> +				sd->shared = *per_cpu_ptr(d.sds, sd_id);
> +				atomic_set(&sd->shared->nr_busy_cpus, sd->span_weight);
> +				atomic_inc(&sd->shared->ref);
> +
> +				/* No SD_NUMA domains. */
> +				if (!parent)
> +					break;
> +
>   				/*
>   				 * For a single LLC per node, allow an
>   				 * imbalance up to 12.5% of the node. This is
> @@ -2641,7 +2651,7 @@ build_sched_domains(const struct cpumask *cpu_map, struct sched_domain_attr *att
>   				 * factors and that there is a correlation
>   				 * between LLCs and memory channels.
>   				 */
> -				nr_llcs = sd->span_weight / child->span_weight;
> +				nr_llcs = parent->span_weight / sd->span_weight;
>   				if (nr_llcs == 1)
>   					imb = sd->span_weight >> 3;
>   				else
> @@ -2650,11 +2660,11 @@ build_sched_domains(const struct cpumask *cpu_map, struct sched_domain_attr *att
>   				sd->imb_numa_nr = imb;
>   
>   				/* Set span based on the first NUMA domain. */
> -				top_p = sd->parent;
> +				top_p = parent;

maybe top_p = parent->parent

thanks,
Chenyu