linux-kernel - Re: [PATCH v6] numa: make node_to_cpumask_map() NUMA_NO

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Date:   Mon, 23 Sep 2019 22:34:10 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Yunsheng Lin <linyunsheng@...wei.com>, catalin.marinas@....com,
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Subject: Re: [PATCH v6] numa: make node_to_cpumask_map() NUMA_NO_NODE aware

On Mon, Sep 23, 2019 at 06:52:35PM +0200, Michal Hocko wrote:
> On Mon 23-09-19 17:48:52, Peter Zijlstra wrote:

> To the NUMA_NO_NODE itself. Your earlier email noted:
> : > +
> : >  	if ((unsigned)node >= nr_node_ids) {
> : >  		printk(KERN_WARNING
> : >  			"cpumask_of_node(%d): (unsigned)node >= nr_node_ids(%u)\n",
> : 
> : I still think this makes absolutely no sense what so ever.
> 
> Did you mean the NUMA_NO_NODE handling or the specific node >= nr_node_ids
> check?

The NUMA_NO_NODE thing. It's is physical impossibility. And if the
device description doesn't give us a node, then the description is
incomplete and wrong and we should bloody well complain about it.

> Because as to NUMA_NO_NODE I believe this makes sense because this is
> the only way that a device is not bound to any numa node.

Which is a physical impossibility.

> I even the
> ACPI standard is considering this optional. Yunsheng Lin has referred to
> the specific part of the standard in one of the earlier discussions.
> Trying to guess the node affinity is worse than providing all CPUs IMHO.

I'm saying the ACPI standard is wrong. Explain to me how it is
physically possible to have a device without NUMA affinity in a NUMA
system?

 1) The fundamental interconnect is not uniform.
 2) The device needs to actually be somewhere.

>From these it seems to follow that access to the device is subject to
NUMA.