Re: [PATCH] PCI: Warn about host bridge device when its numa node is NO_NODE

From: Bjorn Helgaas <helgaas@kernel.org>
To: Robin Murphy <robin.murphy@arm.com>
Cc: Yunsheng Lin <linyunsheng@huawei.com>,
	linux-pci@vger.kernel.org, linux-kernel@vger.kernel.org,
	mhocko@kernel.org, peterz@infradead.org, geert@linux-m68k.org,
	gregkh@linuxfoundation.org, paul.burton@mips.com
Subject: Re: [PATCH] PCI: Warn about host bridge device when its numa node is NO_NODE
Date: Fri, 25 Oct 2019 07:51:47 -0500	[thread overview]
Message-ID: <20191025125147.GA124662@google.com> (raw)
In-Reply-To: <a33e01bd-3054-a2c4-c206-f893e7373e65@arm.com>

On Thu, Oct 24, 2019 at 11:16:41AM +0100, Robin Murphy wrote:
> On 2019-10-23 6:10 pm, Bjorn Helgaas wrote:

> >      PCI: Warn if no host bridge NUMA node info
> >      In pci_call_probe(), we try to run driver probe functions on the node where
> >      the device is attached.  If we don't know which node the device is attached
> >      to, the driver will likely run on the wrong node.  This will still work,
> >      but performance will not be as good as it could be.
> 
> Is it guaranteed to be purely a performance issue? In other words, is there
> definitely no way a physical node could be disabled via idle/hotplug/etc.
> such that unattributed devices can silently disappear while still in use?

I think so.  At least, if it's more than a performance issue, I have
no idea what sort of problem might happen or how to deal with it.

> > @@ -897,6 +897,9 @@ static int pci_register_host_bridge(struct pci_host_bridge *bridge)
> >   	else
> >   		pr_info("PCI host bridge to bus %s\n", name);
> > +	if (nr_node_ids > 1 && pcibus_to_node(bus) == NUMA_NO_NODE)
> > +		dev_warn(&bus->dev, "Unknown NUMA node; performance will be reduced\n");
> 
> I think this still deserves the FW_BUG prefix.

Putting the warning here in pci_register_host_bridge() is convenient
for now but doesn't seem like the ideal place.

I'd rather have the warning at the point where we get the node number,
e.g., in pci_acpi_root_get_node() or of_node_to_nid(), where we would
know what's actually required by spec and we could point to the
specific ACPI device or DT device node that's broken.  Then I think
we'd have a better case for using FW_BUG.

I'm a little hesitant to use FW_BUG here in pci_register_host_bridge()
because we don't know where the node number was supposed to come from,
so we can't reliably determine that the lack of one is a bug.

Bjorn