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

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Date:   Tue, 2 Feb 2021 13:50:20 -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 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 donwtrain 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?

Alex