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

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Date:   Tue, 24 Sep 2019 11:25:51 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Yunsheng Lin <linyunsheng@...wei.com>
Cc:     Michal Hocko <mhocko@...nel.org>, catalin.marinas@....com,
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Subject: Re: [PATCH v6] numa: make node_to_cpumask_map() NUMA_NO_NODE aware

On Tue, Sep 24, 2019 at 09:29:50AM +0800, Yunsheng Lin wrote:
> On 2019/9/24 4:34, Peter Zijlstra wrote:

> > 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 what I can see, NUMA_NO_NODE may make sense in the below case:
> 
> 1) Theoretically, there would be a device that can access all the memory
> uniformly and can be accessed by all cpus uniformly even in a NUMA system.
> Suppose we have two nodes, and the device just sit in the middle of the
> interconnect between the two nodes.
> 
> Even we define a third node solely for the device, we may need to look at
> the node distance to decide the device can be accessed uniformly.
> 
> Or we can decide that the device can be accessed uniformly by setting
> it's node to NUMA_NO_NODE.

This is indeed a theoretical case; it doesn't scale. The moment you're
adding multiple sockets or even board interconnects this all goes out
the window.

And in this case, forcing the device to either node is fine.

> 2) For many virtual deivces, such as tun or loopback netdevice, they
> are also accessed uniformly by all cpus.

Not true; the virtual device will sit in memory local to some node.

And as with physical devices, you probably want at least one (virtual)
queue per node.