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

[Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>]

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@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].
Daniel,

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

Suravee

>
> 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