linux-kernel - Re: Issues with "PCI/LINK: Report degraded links via link bandwidth notification"

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Date:   Tue, 2 Feb 2021 14:25:14 -0600
From:   "Alex G." <mr.nuke.me@...il.com>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     Sinan Kaya <okaya@...nel.org>, Keith Busch <keith.busch@...el.com>,
        Jan Vesely <jano.vesely@...il.com>,
        Lukas Wunner <lukas@...ner.de>,
        Alex Williamson <alex.williamson@...hat.com>,
        Austin Bolen <austin_bolen@...l.com>,
        Shyam Iyer <Shyam_Iyer@...l.com>, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org, Christoph Hellwig <hch@....de>,
        Lucas Stach <l.stach@...gutronix.de>,
        Dave Airlie <airlied@...il.com>,
        Ben Skeggs <skeggsb@...il.com>,
        Alex Deucher <alexdeucher@...il.com>,
        Myron Stowe <myron.stowe@...hat.com>,
        "A. Vladimirov" <vladimirov.atanas@...il.com>
Subject: Re: Issues with "PCI/LINK: Report degraded links via link bandwidth
 notification"

On 2/2/21 2:16 PM, Bjorn Helgaas wrote:
> On Tue, Feb 02, 2021 at 01:50:20PM -0600, Alex G. wrote:
>> On 1/29/21 3:56 PM, Bjorn Helgaas wrote:
>>> On Thu, Jan 28, 2021 at 06:07:36PM -0600, Alex G. wrote:
>>>> On 1/28/21 5:51 PM, Sinan Kaya wrote:
>>>>> On 1/28/2021 6:39 PM, Bjorn Helgaas wrote:
>>>>>> AFAICT, this thread petered out with no resolution.
>>>>>>
>>>>>> If the bandwidth change notifications are important to somebody,
>>>>>> please speak up, preferably with a patch that makes the notifications
>>>>>> disabled by default and adds a parameter to enable them (or some other
>>>>>> strategy that makes sense).
>>>>>>
>>>>>> I think these are potentially useful, so I don't really want to just
>>>>>> revert them, but if nobody thinks these are important enough to fix,
>>>>>> that's a possibility.
>>>>>
>>>>> Hide behind debug or expert option by default? or even mark it as BROKEN
>>>>> until someone fixes it?
>>>>>
>>>> Instead of making it a config option, wouldn't it be better as a kernel
>>>> parameter? People encountering this seem quite competent in passing kernel
>>>> arguments, so having a "pcie_bw_notification=off" would solve their
>>>> problems.
>>>
>>> I don't want people to have to discover a parameter to solve issues.
>>> If there's a parameter, notification should default to off, and people
>>> who want notification should supply a parameter to enable it.  Same
>>> thing for the sysfs idea.
>>
>> I can imagine cases where a per-port flag would be useful. For example, a
>> machine with a NIC and a couple of PCIe storage drives. In this example, the
>> PCIe drives downtrain willie-nillie, so it's useful to turn off their
>> notifications, but the NIC absolutely must not downtrain. It's debatable
>> whether it should be default on or default off.
>>
>>> I think we really just need to figure out what's going on.  Then it
>>> should be clearer how to handle it.  I'm not really in a position to
>>> debug the root cause since I don't have the hardware or the time.
>>
>> I wonder
>> (a) if some PCIe devices are downtraining willie-nillie to save power
>> (b) if this willie-nillie downtraining somehow violates PCIe spec
>> (c) what is the official behavior when downtraining is intentional
>>
>> My theory is: YES, YES, ASPM. But I don't know how to figure this out
>> without having the problem hardware in hand.
>>
>>> If nobody can figure out what's going on, I think we'll have to make it
>>> disabled by default.
>>
>> I think most distros do "CONFIG_PCIE_BW is not set". Is that not true?
> 
> I think it *is* true that distros do not enable CONFIG_PCIE_BW.
> 
> But it's perfectly reasonable for people building their own kernels to
> enable it.  It should be safe to enable all config options.  If they
> do enable CONFIG_PCIE_BW, I don't want them to waste time debugging
> messages they don't expect.
> 
> If we understood why these happen and could filter out the expected
> ones, that would be great.  But we don't.  We've already wasted quite
> a bit of Jan's and Atanas' time, and no doubt others who haven't
> bothered to file bug reports.
> 
> So I think I'll queue up a patch to remove the functionality for now.
> It's easily restored if somebody debugs the problem or adds a
> command-line switch or something.

I think it's best we make it a module (or kernel) parameter, default=off 
for the time being.

Alex