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

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Message-ID: <CAErSpo4x+=EQ5h8DuzoRznZ_d8Roy4LHer7ELEVeHxDsKwDhzA@mail.gmail.com>
Date:	Fri, 21 Mar 2014 10:11:31 -0600
From:	Bjorn Helgaas <bhelgaas@...gle.com>
To:	Daniel J Blueman <daniel@...ascale.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	"x86@...nel.org" <x86@...nel.org>, Borislav Petkov <bp@...e.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Steffen Persvold <sp@...ascale.com>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
	kim.naru@....com,
	Aravind Gopalakrishnan <aravind.gopalakrishnan@....com>,
	Myron Stowe <myron.stowe@...hat.com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	"linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>
Subject: Re: [PATCH] Fix northbridge quirk to assign correct NUMA node

[+cc Rafael, linux-acpi for _PXM questions]

On Thu, Mar 20, 2014 at 9:38 PM, Daniel J Blueman <daniel@...ascale.com> wrote:
> On 21/03/2014 06:07, Bjorn Helgaas wrote:
>> On Thu, Mar 13, 2014 at 5:43 AM, Daniel J Blueman <daniel@...ascale.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.

>> 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.
> ...
> 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.

I'm confused about which devices we're talking about.  We currently
look at _PXM for PNP0A08 (and PNP0A03) ACPI devices.  The resulting
node is associated with every PCI device we enumerate below the
PNP0A08 bridge.  This association is made in pci_device_add().

When you say "northbridge PCI devices should be associated with their
actual NUMA node," I assume you mean the 00:18.x and 00:19.x devices
("AMD Family 10h Processor ..."), since those seem to be what the
quirk applies to.  You are *not* talking about 00:00.0 ("ATI RD890
Northbridge"), right?

You mention irqbalance; is the NUMA node information for the 00:18.x
and 00:19.x devices important because you get a lot of interrupts from
those devices?  Or is the issue with actual I/O devices (NICs, SCSI
adapters, etc.)?  If so, I don't see how this quirk would affect
those, because the node information for them comes from the PNP0A08
bridge (in pci_device_add()), not from the 00:00.0, 00:18.x, or
00:19.x devices.

> 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.

ACPI allows _PXM for any device, so this might be a possible approach.
 However, it looks like Linux only pays attention to _PXM for
PNP0A08/03, CPUs, memory and IOAPICs (which seems like a Linux defect
to me).

I'm really worried about the approach here:

        pci_read_config_dword(nb_ht, 0x60, &val);
        node = pcibus_to_node(dev->bus) | (val & 7);

because the pcibus_to_node() information comes indirectly from _PXM,
and the "val" part comes from the hardware, and I don't think these
are the same node number space.  If I understand correctly, the BIOS
can synthesize whatever numbers it wants for _PXM, which returns a
"proximity domain," and then Linux can make up its own mapping of
"proximity domain" to "logical Linux node."  So I don't see why we can
assume that it's valid to OR in the bits from a PCI config register to
this logical Linux node number.

> [1] http://quora.org/2014/lspci.txt
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