Subject: Re: [RFC PATCH] powerpc/numa: add ability to disable and debug topology updates

From: Nathan Fontenot <[email-masked]>
Date: Tue, 16 Sep 2014 14:42:20 -0500

On 09/09/2014 03:09 PM, Nishanth Aravamudan wrote:
> We have hit a few customer issues with the topology update code (VPHN
> and PRRN). It would be nice to be able to debug the notifications coming
> from the hypervisor in both cases to the LPAR, as well as to disable
> reacting to the notifications, to narrow down the source of the
> problems. Add a basic level of such functionality, similar to the numa=
> command-line parameter.
> 
> Signed-off-by: Nishanth Aravamudan <[email-masked]>
> ---
> This is pretty rough, but has been useful in the field already. I'm not
> sure if more information would be useful than this basic amount.
> 
> diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
> index 5ae8608ca9f5..6e3b9e3a2ab4 100644
> --- a/Documentation/kernel-parameters.txt
> +++ b/Documentation/kernel-parameters.txt
> @@ -3370,6 +3370,13 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
>  			e.g. base its process migration decisions on it.
>  			Default is on.
>  
> +	topology_updates= [KNL, PPC, NUMA]
> +			Format: {off | debug}
> +			Specify if the kernel should ignore (off) or
> +			emit more information (debug) when the
> +			hypervisor sends NUMA topology updates to an
> +			LPAR.
> +
>  	tp720=		[HW,PS2]
>  
>  	tpm_suspend_pcr=[HW,TPM]
> diff --git a/arch/powerpc/mm/numa.c b/arch/powerpc/mm/numa.c
> index d7737a542fd7..72c5ad313cbe 100644
> --- a/arch/powerpc/mm/numa.c
> +++ b/arch/powerpc/mm/numa.c
> @@ -1160,6 +1160,28 @@ static int __init early_numa(char *p)
>  }
>  early_param("numa", early_numa);
>  
> +static int topology_updates_enabled = 1;
> +static int topology_updates_debug = 0;
> +
> +static int __init early_topology_updates(char *p)
> +{
> +	if (!p)
> +		return 0;
> +
> +	if (strstr(p, "off")) {
> +		printk(KERN_INFO "Disabling topology updates\n");
> +		topology_updates_enabled = 0;
> +	}
> +
> +	if (strstr(p, "debug")) {
> +		printk(KERN_INFO "Enabling topology updates debug\n");
> +		topology_updates_debug = 1;
> +	}
> +
> +	return 0;
> +}
> +early_param("topology_updates", early_topology_updates);
> +
>  #ifdef CONFIG_MEMORY_HOTPLUG
>  /*
>   * Find the node associated with a hot added memory section for
> @@ -1546,6 +1568,9 @@ int arch_update_cpu_topology(void)
>  	struct device *dev;
>  	int weight, new_nid, i = 0;
>  
> +	if (!topology_updates_enabled)
> +		return 0;
> +
>  	weight = cpumask_weight(&cpu_associativity_changes_mask);
>  	if (!weight)
>  		return 0;
> @@ -1610,6 +1635,25 @@ int arch_update_cpu_topology(void)
>  	 *
>  	 * And for the similar reason, we will skip all the following updating.
>  	 */
> +
> +	if (topology_updates_debug) {
> +		char *buf = kmalloc_array(NR_CPUS*5, sizeof(char), GFP_KERNEL);
> +		cpumask_scnprintf(buf, NR_CPUS*5, &updated_cpus);
> +		printk(KERN_DEBUG "Topology update for the following CPUs:\n");
> +		printk(KERN_DEBUG " %s\n", buf);
> +		printk(KERN_DEBUG "cpumask_weight(&updated_cpus)) = %u\n",
> +						cpumask_weight(&updated_cpus));
> +
> +		if (cpumask_weight(&updated_cpus)) {
> +			for (ud = &updates[0]; ud; ud = ud->next) {
> +				printk(KERN_DEBUG "cpu %d moving from node %d "
> +						  "to %d\n", ud->cpu,
> +						  ud->old_nid, ud->new_nid);
> +			}
> +		}
> +		kfree(buf);
> +	}
> +
>  	if (!cpumask_weight(&updated_cpus))
>  		goto out;
>  
> @@ -1807,8 +1851,10 @@ static const struct file_operations topology_ops = {
>  
>  static int topology_update_init(void)
>  {
> -	start_topology_update();
> -	proc_create("powerpc/topology_updates", 0644, NULL, &topology_ops);
> +	if (topology_updates_enabled) {
> +		start_topology_update();
> +		proc_create("powerpc/topology_updates", 0644, NULL, &topology_ops);
> +	}
>  

Is there any reason you would want to enable topology updates at some
later point?

If so you could still create the /proc file and update it to set
topology_updates_enabled appropriately in start_topology_update()
and stop_topology_update().

-Nathan

>  	return 0;
>  }
> 
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