linux-kernel - Re: [PATCH 2/2] powerpc/hotplug: Ensure nodes initialized for hotplug

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Date:   Mon, 16 Oct 2017 23:54:39 +1100
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Michael Bringmann <mwb@...ux.vnet.ibm.com>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Cc:     Michael Bringmann <mwb@...ux.vnet.ibm.com>,
        John Allen <jallen@...ux.vnet.ibm.com>,
        Nathan Fontenot <nfont@...ux.vnet.ibm.com>
Subject: Re: [PATCH 2/2] powerpc/hotplug: Ensure nodes initialized for hotplug

Michael Bringmann <mwb@...ux.vnet.ibm.com> writes:

> powerpc/hotplug: On systems like PowerPC which allow 'hot-add' of CPU,
> it may occur that the new resources are to be inserted into nodes
> that were not used for memory resources at bootup.  Many different
> configurations of PowerPC resources may need to be supported depending
> upon the environment.

Give me some detail please?!

> This patch fixes some problems encountered at

What problems?

> runtime with configurations that support memory-less nodes, but which
> allow CPUs to be added at and after boot.

How does it fix those problems?

> diff --git a/arch/powerpc/mm/numa.c b/arch/powerpc/mm/numa.c
> index b385cd0..e811dd1 100644
> --- a/arch/powerpc/mm/numa.c
> +++ b/arch/powerpc/mm/numa.c
> @@ -1325,6 +1325,17 @@ static long vphn_get_associativity(unsigned long cpu,
>  	return rc;
>  }
>  
> +static int verify_node_preparation(int nid)
> +{

I would not expect a function called "verify" ...

> +	if ((NODE_DATA(nid) == NULL) ||
> +	    (NODE_DATA(nid)->node_spanned_pages == 0)) {
> +		if (try_online_node(nid))

.. to do something like online a node.

> +			return first_online_node;
> +	}
> +
> +	return nid;
> +}
> +
>  /*
>   * Update the CPU maps and sysfs entries for a single CPU when its NUMA
>   * characteristics change. This function doesn't perform any locking and is
> @@ -1433,9 +1444,11 @@ int numa_update_cpu_topology(bool cpus_locked)
>  		/* Use associativity from first thread for all siblings */
>  		vphn_get_associativity(cpu, associativity);
>  		new_nid = associativity_to_nid(associativity);
> -		if (new_nid < 0 || !node_online(new_nid))
> +		if (new_nid < 0 || !node_possible(new_nid))
>  			new_nid = first_online_node;
>  
> +		new_nid = verify_node_preparation(new_nid);

You're being called part-way through CPU hotplug here, are we sure it's
safe to go and do memory hotplug from there? What's the locking
situation?

cheers