On Tue, Sep 03, 2019 at 02:19:04PM +0800, Yunsheng Lin wrote:

On 2019/9/2 20:56, Peter Zijlstra wrote:

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On Mon, Sep 02, 2019 at 08:25:24PM +0800, Yunsheng Lin wrote:

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On 2019/9/2 15:25, Peter Zijlstra wrote:

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On Mon, Sep 02, 2019 at 01:46:51PM +0800, Yunsheng Lin wrote:

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On 2019/9/1 0:12, Peter Zijlstra wrote:

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1) because even it is not set, the device really does belong to a node.
It is impossible a device will have magic uniform access to memory when
CPUs cannot.

So it means dev_to_node() will return either NUMA_NO_NODE or a
valid node id?

NUMA_NO_NODE := -1, which is not a valid node number. It is also, like I
said, not a valid device location on a NUMA system.

Just because ACPI/BIOS is shit, doesn't mean the device doesn't have a
node association. It just means we don't know and might have to guess.

How do we guess the device's location when ACPI/BIOS does not set it?

See device_add(), it looks to the device's parent and on NO_NODE, puts
it there.

Lacking any hints, just stick it to node0 and print a FW_BUG or
something.

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It seems dev_to_node() does not do anything about that and leave the
job to the caller or whatever function that get called with its return
value, such as cpumask_of_node().

Well, dev_to_node() doesn't do anything; nor should it. It are the
callers of set_dev_node() that should be taking care.

Also note how device_add() sets the device node to the parent device's
node on NUMA_NO_NODE. Arguably we should change it to complain when it
finds NUMA_NO_NODE and !parent.

Is it possible that the node id set by device_add() become invalid
if the node is offlined, then dev_to_node() may return a invalid
node id.

In that case I would expect the device to go away too. Once the memory
controller goes away, the PCI bus connected to it cannot continue to
function.

From the comment in select_fallback_rq(), it seems that a node can
be offlined, not sure if node offline process has taken cared of that?

	/*
         * If the node that the CPU is on has been offlined, cpu_to_node()
         * will return -1. There is no CPU on the node, and we should
         * select the CPU on the other node.
         */

Ugh, so I disagree with that notion. cpu_to_node() mapping should be
fixed, you simply cannot change it after boot, too much stuff relies on
it.

Setting cpu_to_node to -1 on node offline is just wrong. But alas, it
seems this is already so.

With the above assumption that a device is always on a valid node,
the node id returned from dev_to_node() can be safely passed to
cpumask_of_node() without any checking?