linux-kernel - Re: [PATCH v3] pseries: prevent free CPU ids to be reused on another node

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Message-ID: <87blaqkuty.fsf@linux.ibm.com>
Date:   Wed, 07 Apr 2021 09:55:37 -0500
From:   Nathan Lynch <nathanl@...ux.ibm.com>
To:     Laurent Dufour <ldufour@...ux.ibm.com>
Cc:     cheloha@...ux.ibm.com, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org, mpe@...erman.id.au,
        benh@...nel.crashing.org, paulus@...ba.org
Subject: Re: [PATCH v3] pseries: prevent free CPU ids to be reused on
 another node

Laurent Dufour <ldufour@...ux.ibm.com> writes:
> Changes since V2, addressing Nathan's comments:
>  - Remove the retry feature
>  - Reduce the number of local variables (removing 'i')

I was more interested in not having two variables for NUMA nodes in the
function named 'node' and 'nid', hoping at least one of them could have
a more descriptive name. See below.

>  static int pseries_add_processor(struct device_node *np)
>  {
> -	unsigned int cpu;
> +	unsigned int cpu, node;
>  	cpumask_var_t candidate_mask, tmp;
> -	int err = -ENOSPC, len, nthreads, i;
> +	int err = -ENOSPC, len, nthreads, nid;
>  	const __be32 *intserv;
>  
>  	intserv = of_get_property(np, "ibm,ppc-interrupt-server#s", &len);
> @@ -163,9 +169,17 @@ static int pseries_add_processor(struct device_node *np)
>  	zalloc_cpumask_var(&candidate_mask, GFP_KERNEL);
>  	zalloc_cpumask_var(&tmp, GFP_KERNEL);
>  
> +	/*
> +	 * Fetch from the DT nodes read by dlpar_configure_connector() the NUMA
> +	 * node id the added CPU belongs to.
> +	 */
> +	nid = of_node_to_nid(np);
> +	if (nid < 0 || !node_possible(nid))
> +		nid = first_online_node;
> +
>  	nthreads = len / sizeof(u32);
> -	for (i = 0; i < nthreads; i++)
> -		cpumask_set_cpu(i, tmp);
> +	for (cpu = 0; cpu < nthreads; cpu++)
> +		cpumask_set_cpu(cpu, tmp);
>  
>  	cpu_maps_update_begin();
>  
> @@ -173,6 +187,19 @@ static int pseries_add_processor(struct device_node *np)
>  
>  	/* Get a bitmap of unoccupied slots. */
>  	cpumask_xor(candidate_mask, cpu_possible_mask, cpu_present_mask);
> +
> +	/*
> +	 * Remove free ids previously assigned on the other nodes. We can walk
> +	 * only online nodes because once a node became online it is not turned
> +	 * offlined back.
> +	 */
> +	for_each_online_node(node) {
> +		if (node == nid) /* Keep our node's recorded ids */
> +			continue;
> +		cpumask_andnot(candidate_mask, candidate_mask,
> +			       node_recorded_ids_map[node]);
> +	}
> +

e.g. change 'nid' to 'assigned_node' or similar, and I think this
becomes easier to follow.

Otherwise the patch looks fine to me now.