Message-ID: <543FFA85.70908@redhat.com>
Date:	Thu, 16 Oct 2014 10:04:05 -0700
From:	Alexander Duyck <alexander.h.duyck@...hat.com>
To:	Prarit Bhargava <prarit@...hat.com>
CC:	Bjorn Helgaas <bhelgaas@...gle.com>,
	"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 09:07 AM, Prarit Bhargava wrote:
> On 10/16/2014 10:44 AM, Alexander Duyck wrote:
>> 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.
> Yes ...
>
>> I suspect that this driver doesn't even take that into account as it was likely
>> only written for Sandy Bridge architectures.
>>
> Nope.  New hardware.  The issue is that there is only a performance impact if
> local node memory is used, o/w the claim is that the performance drops to that
> of doing software crypto.

Yes, but the problem is what is considered a local node?  In the case of 
the Nehalem architecture it wasn't uncommon to have two sockets share an 
IOH.  As a result each node would have its own memory, but the PCI 
devices were shared.  That platform would fail this test, but each node 
has its own memory and I suspect the QPI overhead wouldn't be that bad 
since the link is dedicated to the IOH connection.

What I am getting at is that this driver was likely intended for 
specifically the current and follow-on generations of Xeon processors.  
As such it deciding to reject all NUMA_NO_NODE configuration may be 
acceptable for it since they have a very specific architecture and CPU 
in mind.  For the workaround if you are going to try and do something as 
generic, you will need to be mindful of how the other architectures deal 
with NUMA and PCI.

>>>>>>> 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.
>>
> I was thinking that I could modify the patch to do this, but I'd rather
> investigate Bjorn's suggestion first.  I think his approach has some merits, but
> I will definitely TAINT if I go with that approach too.

If we are doing any sort of workaround we should mark the system as 
being tainted.  Otherwise we are just going to let whatever workaround 
you implement be accepted and nobody will ever bother to fix the BIOS in 
this generation or the next.

>>>> 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.
>>
> Just a couple of printks to the screen.

Yeah, taint would definitely be preferred.  Then if something has a 
panic later because we didn't work around this correctly we should be 
able to see that this is a tainted system in the panic dump.  I did the 
same thing for DSA.

>> 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.
> The problem is NUMA_NO_NODE, which as you point out can be a valid
> configuration.  So in some cases system designers may have intentionally done
> this (Can anyone think of a valid reason to leave off the _PXM, or have it
> assigned to NUMA_NO_NODE?), so the previous statement about having an opt-in,
> then attempting to calculate the node location, and now TAINTING might be a good
> direction to move in.

That is my thought.  We would just need to come up with a good way of 
sorting these platforms out and that can be the challenge.  From past 
experience I have even seen some systems where the memory layout wasn't 
configured correctly in the BIOS/ACPI tables so this is pretty much just 
a crapshoot as to if we can figure out the actual layout or not via 
software.  Maybe this could be identified with 
TAINT_OVERRIDDEN_ACPI_TABLE since we are having to hand code an ACPI table.

> OTOH ... what do we do about older unsupported hardware that won't have new BIOS
> releases?  Those would basically say "Go fix your BIOS" and there's nothing that
> could be done :/.  All those users see is a loud warning...

I'm not saying we cannot provide a workaround for it, but at the same 
time as you have pointed out this is mostly just a performance issue due 
to the BIOS and we could be introducing new issues by trying to fix it 
in the kernel.

The loud warning is both for the users and those of us that are stuck 
debugging the issues they find when their platform doesn't run as 
expected and triggers a panic.  You have to keep in mind that by working 
around things we run the risk of introducing new issues and we have to 
know when we have entered one of those cases.

>>>> 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.
> Well ... let me think about this for a bit.  The big issue is what happens
> during socket hot-add events and the PCI root bridges that are added at that
> time.  It may not be possible to come up with a correct calculation :( but let
> me give it a shot.  IIRC the node-to-socket map should be static...
>
> P.

I don't know if there is a good way to resolve all this programmatically 
since I have seen a number of issues where even the NUMA memory layout 
is messed up.  The sysfs might be a good approach however I would 
suggest tainting the kernel at that point as you are overwritting 
pre-defined BIOS/ACPI table values and that may have certain side 
effects if things don't get configured correctly.

Thanks,

Alex
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