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

[Daniel J Blueman <daniel@numascale.com>]

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@numascale.com> 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 invarient
>> on most systems.
>>
>> Tested on fam10h and fam15h single and multi-fabric systems and candidate
>> 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.

Noted.

> 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].

With _PXM, the rest of the PCI bus hierarchy has the right NUMA node 
associated, but the northbridge PCI devices should be associated with 
their actual NUMA node, 0, 1, 2, 3 for the first server in this example. 
The quirk fixes this up; irqbalance at least uses this NUMA data exposed 
in /sys.

The alternative to the quirk may be to explicitly express the 
northbridge PCI devices in the AML with their own _PXM methods. If it's 
valid, it may be the honest approach, though the quirk may be needed for 
most BIOSs; I can check the AML on a few servers to confirm if helpful.

Thanks,
   Daniel

[1] http://quora.org/2014/lspci.txt
-- 
Daniel J Blueman
Principal Software Engineer, Numascale