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

[Alex G.]

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