Date:	Thu, 16 Oct 2014 07:44:02 -0700
From:	Alexander Duyck <alexander.h.duyck@...hat.com>
To:	Prarit Bhargava <prarit@...hat.com>,
	Bjorn Helgaas <bhelgaas@...gle.com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Myron Stowe <mstowe@...hat.com>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>
Subject: Re: [PATCH] pci, add sysfs numa_node write function


On 10/16/2014 05:32 AM, Prarit Bhargava wrote:
> On 10/15/2014 05:20 PM, Bjorn Helgaas wrote:
>> On Wed, Oct 15, 2014 at 1:47 PM, Prarit Bhargava <prarit@...hat.com> wrote:
>>> On 10/15/2014 03:23 PM, Bjorn Helgaas wrote:
>>>> Hi Prarit,
>>>>
>>>> On Wed, Oct 15, 2014 at 1:05 PM, Prarit Bhargava <prarit@...hat.com> wrote:
>>>>> Consider a multi-node, multiple pci root bridge system which can be
>>>>> configured into one large node or one node/socket.  When configuring the
>>>>> system the numa_node value for each PCI root bridge is always set
>>>>> incorrectly to -1, or NUMA_NO_NODE, rather than to the node value of each
>>>>> socket.  Each PCI device inherits the numa value directly from it's parent
>>>>> device, so that the NUMA_NO_NODE value is passed through the entire PCI
>>>>> tree.
>>>>>
>>>>> Some new drivers, such as the Intel QAT driver, drivers/crypto/qat,
>>>>> require that a specific node be assigned to the device in order to
>>>>> achieve maximum performance for the device, and will fail to load if the
>>>>> device has NUMA_NO_NODE.
>>>> It seems ... unfriendly for a driver to fail to load just because it
>>>> can't guarantee maximum performance.  Out of curiosity, where does
>>>> this actually happen?  I had a quick look for NUMA_NO_NODE and
>>>> module_init() functions in drivers/crypto/qat, and I didn't see the
>>>> spot.
>>> The whole point of the Intel QAT driver is to guarantee max performance.  If
>>> that is not possible the driver should not load (according to the thread
>>> mentioned below)

This is just short-sighted thinking.  The fact that the PCI device 
advertises -1 means that either the BIOS isn't configured, or the PCI 
slots are shared as was the case on some Nehalem systems where the IOH 
was shared between two sockets.  I suspect that this driver doesn't even 
take that into account as it was likely only written for Sandy Bridge 
architectures.

>>>>> To use this, one can do
>>>>>
>>>>> echo 3 > /sys/devices/pci0000:ff/0000:ff:1f.3/numa_node
>>>>>
>>>>> to set the numa node for PCI device 0000:ff:1f.3.
>>>> It definitely seems wrong that we don't set the node number correctly.
>>>> pci_acpi_scan_root() sets the node number by looking for a _PXM method
>>>> that applies to the host bridge.  Why does that not work in this case?
>>>>   Does the BIOS not supply _PXM?
>>> Yeah ... unfortunately the BIOS is broken in this case.  And I know what you're
>>> thinking ;) -- why not get the BIOS fixed?  I'm through relying on BIOS fixes
>>> which can take six months to a year to appear in a production version... I've
>>> been bitten too many times by promises of BIOS fixes that never materialize.
>> Yep, I understand.  The question is how we implement a workaround so
>> it doesn't become the accepted way to do things.  Obviously we don't
>> want people manually grubbing through numactl/lspci output or writing
>> shell scripts to do things that *should* happen automatically.

I'd say if nothing else we should flag the system as tainted as soon as 
we start overwriting BIOS/ACPI configured values with sysfs. This is one 
of the reasons for the TAINT_FIRMWARE_WORKAROUND even existing.

>> Somewhere in the picture there needs to be a feedback loop that
>> encourages the vendor to fix the problem.  I don't see that happening
>> yet.  Having QAT fail because the platform didn't supply the
>> information required to make it work would be a nice loop.  I don't
>> want to completely paper over the problem without providing some other
>> kind of feedback at the same time.
> Okay -- I see what you're after here and I completely agree with it.  But
> sometimes I feel like I banging on a silent drum with some of these companies
> about this stuff :(  My frustration with these companies is starting to show I
> guess...

Just how visible is the QAT driver load failure?  I has a similar issue 
with DCA not being configured in a number of BIOSes and it wasn't until 
I made the issue painfully visible with TAINT_FIRMWARE_WORKAROUND that I 
started to see any traction on getting this fixed in the BIOSes.

We would need to sort out the systems that actually have bad BIOSes 
versus just being configured without PCI slots directly associated with 
any given NUMA node since there are systems where that is a valid 
configuration.

>> You're probably aware of [1], which was the same problem.  Apparently
>> it was originally reported to RedHat as [2] (which is private, so I
>> can't read it).  That led to a workaround hack for some AMD systems
>> [3, 4].
> Yeah ... part of me was thinking that maybe I should do something like
> the above but I didn't know how you'd feel about expanding that hack.  I'll look
> into it.  I'd prefer it to be opt-in with a kernel parameter.
>
> P.

Are you thinking something like a "pci=assign-numa"?  The problem is 
there doesn't seem to be a good way to currently determine the NUMA 
layout without the information being provided by the BIOS/ACPI tables, 
and we probably don't want to be creating a definition of the NUMA 
layout per platform.

Thanks,

Alex

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