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

From: Daniel J Blueman
Date: Fri Mar 21 2014 - 00:15:58 EST

On 21/03/2014 11:51, Suravee Suthikulpanit wrote:

On a typical AMD system, there are two types of host bridges:
* PCI Root Complex Host bridge (e.g. RD890, SR56xx, etc.)
* CPU Host bridge

Here is an example from a 2 sockets system:

$ lspci

The host bridge 00:00.0 is basically the PCI root complex which connects
to the actual PCI bus with
PCI devices hanging off of it. However, the host bridge 00:[18,19].x
are the CPU host bridges,
each of which represents a CPU node within the system. In system with
single root complex,
the root complex is normally connected to node 0 (i.e. 00:18.0) via
non-coherent HT (I/O) link.

Even though the CPU host bridge 00:[18,19].x is on the same bus as the
PCI root complex, it should
not be using the NUMA information from the PCI root complex host bridge.

This is unavoidable unless we special-case it via another mechanism (ie not quirks), since the northbridges/CPU host bridges are logically under the _PXM method.

Therefore, I don't think we should be using the pcibus_to_node(dev->bus)
Only the "val" from pci_read_config_dword(nb_ht, 0x60, &val), should be
used here.

Using only effectively the NUMA node ID (HT node ID here) would associate all the northbridges with the first fabric, which is false information. If there was no quirk, they'd all be associated with the first NUMA node in each fabric, as you'd expect.

This was the only safe and defensible one-liner approach I could prepare; if you find it introduces a regression or you can find a better approach, do tell. If not, we can decouple this fix from an overall new approach, since it's unlikely that'll get backported to stable kernels.


On 3/20/2014 5:07 PM, 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
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?

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.


Signed-off-by: Daniel J Blueman <daniel@xxxxxxxxxxxxx>
Acked-by: Steffen Persvold <sp@xxxxxxxxxxxxx>
arch/x86/kernel/quirks.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/quirks.c b/arch/x86/kernel/quirks.c
index 04ee1e2..52dbf1e 100644
--- a/arch/x86/kernel/quirks.c
+++ b/arch/x86/kernel/quirks.c
@@ -529,7 +529,7 @@ static void quirk_amd_nb_node(struct pci_dev *dev)

pci_read_config_dword(nb_ht, 0x60, &val);
- node = val & 7;
+ node = pcibus_to_node(dev->bus) | (val & 7);
* Some hardware may return an invalid node ID,
* so check it first:

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Daniel J Blueman
Principal Software Engineer, Numascale
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