RE: [PATCH v7] lib/group_cpus: Snapshot cluster masks to keep grouping hotplug invariant

[Guo, Wangyang]

On Wed, 11 Nov 2026 01:46:11, Vishal Badole <Vishal.Badole@xxxxxxx> wrote:
>  static int alloc_cluster_groups(unsigned int ncpus,
>  				unsigned int ngroups,
>  				struct cpumask *node_cpumask,
> @@ -299,6 +380,9 @@ static int alloc_cluster_groups(unsigned int ncpus,
>  	const struct cpumask **clusters;
>  	struct node_groups *cluster_groups;
>
> +	if (!cluster_snapshot)
> +		goto no_cluster;
> +
>  	cpumask_copy(msk, node_cpumask);
>
>  	/* Probe how many clusters in this node. */
> @@ -307,7 +391,7 @@ static int alloc_cluster_groups(unsigned int ncpus,
>  		if (cpu >= nr_cpu_ids)
>  			break;
>
> -		cluster_mask = topology_cluster_cpumask(cpu);
> +		cluster_mask = cluster_snapshot[cpu];
>  		if (!cpumask_weight(cluster_mask))
>  			goto no_cluster;
>  		/* Clean out CPUs on the same cluster. */
> @@ -322,7 +406,7 @@ static int alloc_cluster_groups(unsigned int ncpus,
>  	/* Allocate memory based on cluster number. */
>  	clusters = kzalloc_objs(*clusters, ncluster);
>  	if (!clusters)
> -		goto no_cluster;
> +		return -ENOMEM;
>  	cluster_groups = kzalloc_objs(struct node_groups, ncluster);
>  	if (!cluster_groups)
>  		goto fail_cluster_groups;
> @@ -331,7 +415,7 @@ static int alloc_cluster_groups(unsigned int ncpus,
>  	cpumask_copy(msk, node_cpumask);
>  	for (n = 0; n < ncluster; n++) {
>  		cpu = cpumask_first(msk);
> -		cluster_mask = topology_cluster_cpumask(cpu);
> +		cluster_mask = cluster_snapshot[cpu];
>  		nc = cpumask_weight_and(cluster_mask, node_cpumask);
>  		clusters[n] = cluster_mask;
>  		cluster_groups[n].id = n;
> @@ -347,6 +431,7 @@ static int alloc_cluster_groups(unsigned int ncpus,
>
>   fail_cluster_groups:
>  	kfree(clusters);
> +	return -ENOMEM;
>   no_cluster:
>  	return 0;
>  }
> @@ -354,9 +439,9 @@ static int alloc_cluster_groups(unsigned int ncpus,
>  /*
>   * Try group CPUs evenly for cluster locality within a NUMA node.
>   *
> - * Return: true if success, false otherwise.
> + * Return: cluster count on success, 0 if inapplicable, or -ENOMEM on failure.
>   */
> -static bool __try_group_cluster_cpus(unsigned int ncpus,
> +static int __try_group_cluster_cpus(unsigned int ncpus,
>  				     unsigned int ngroups,
>  				     struct cpumask *node_cpumask,
>  				     struct cpumask *masks,
> @@ -365,18 +450,17 @@ static bool __try_group_cluster_cpus(unsigned int ncpus,
>  {
>  	struct node_groups *cluster_groups;
>  	const struct cpumask **clusters;
> -	unsigned int ncluster;
> -	bool ret = false;
> +	int ncluster;
>  	cpumask_var_t nmsk;
>  	unsigned int i, nc;

If we could move "if (!cluster_snapshot)" from alloc_cluster_groups() to
here, the unnecessary cpumask alloc/free could be avoid if snapshot if
unavailable.

>  	if (!zalloc_cpumask_var(&nmsk, GFP_KERNEL))
> -		goto fail_nmsk_alloc;
> +		return -ENOMEM;
>
>  	ncluster = alloc_cluster_groups(ncpus, ngroups, node_cpumask, nmsk,
>  					&clusters, &cluster_groups);
>
> -	if (ncluster == 0)
> +	if (ncluster <= 0)
>  		goto fail_no_clusters;
>
>  	for (i = 0; i < ncluster; i++) {
> @@ -392,13 +476,11 @@ static bool __try_group_cluster_cpus(unsigned int ncpus,
>  		assign_cpus_to_groups(nc, nmsk, nv, masks, curgrp, last_grp);
>  	}
>
> -	ret = true;
>  	kfree(cluster_groups);
>  	kfree(clusters);
>   fail_no_clusters:
>  	free_cpumask_var(nmsk);
> - fail_nmsk_alloc:
> -	return ret;
> +	return ncluster;
>  }
>