Re: [PATCH] Fix northbridge quirk to assign correct NUMA node

From: Daniel J Blueman
Date: Sun Mar 23 2014 - 10:30:43 EST

On 03/22/2014 01:16 AM, Suravee Suthikulpanit wrote:
On 3/20/2014 10:38 PM, Daniel J Blueman wrote:
On 21/03/2014 06:07, Bjorn Helgaas wrote:
[+cc linux-pci, Myron, Suravee, Kim, Aravind]

On Thu, Mar 13, 2014 at 5:43 AM, Daniel J Blueman
<daniel@xxxxxxxxxxxxx> wrote:
For systems with multiple servers and routed fabric, all
northbridges get
assigned to the first server. Fix this by also using the node
reported from
the PCI bus. For single-fabric systems, the northbriges are on PCI
bus 0
by definition, which are on NUMA node 0 by definition, so this is
on most systems.

Tested on fam10h and fam15h single and multi-fabric systems and
for stable.

I wish this had been cc'd to linux-pci. We're talking about a related
change by Suravee there. In fact, we were hoping this quirk could be
removed altogether.


I don't understand what this quirk is doing. Normally we discover the
NUMA node for a PCI host bridge via the ACPI _PXM method. The way
_PXM works is that every PCI device in the hierarchy below the bridge
inherits the same node number as the host bridge. I first thought
this might be a workaround for a system that lacks _PXM, but I don't
think that can be right, because you're only changing the node for a
few devices, not the whole hierarchy.
So I suspect the problem is more complicated, and maybe _PXM is
insufficient to describe the topology? Are there subtrees that should
have nodes different from the host bridge?

Yes; see below.

I know this patch is already in v3.14-rc7, but I'd still like to
understand it so we can do the right thing with Suravee's patch.

The _PXM method associates each northbridge with the first NUMA node,
0 in single-fabric systems, and eg 4 for the second server in a
multi-fabric system with 2 dual-module Opterons (with 2 NUMA nodes
internally) etc, since the northbridges appear in the
PCI tree, under the host bridge, not above it [1].

That lspci looks interesting, what is the value returned from
pci_bus_to_node() on your system for each fabric?

pci_bus_to_node returns 0 for PCI domain 0000, 2 for PCI domain 0001, 4 for PCI domain 0002 and so on.

Our processor fabric interconnect has HyperTransport NodeId 2 on each server (as they start from bus 0, device 0x18 of course):
0000:00:1a.0 Host bridge: Device 1b47:0601 (rev 02)
0000:00:1a.1 Host bridge: Device 1b47:0602 (rev 02)

Daniel J Blueman
Principal Software Engineer, Numascale
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