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

[Alex G.]

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