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

[Alex Deucher]

On Thu, Jan 28, 2021 at 6:39 PM Bjorn Helgaas <helgaas@xxxxxxxxxx> wrote:
>
> [+cc Atanas -- thank you very much for the bug report!]
>
> On Sat, Feb 22, 2020 at 10:58:40AM -0600, Bjorn Helgaas wrote:
> > On Wed, Jan 15, 2020 at 04:10:08PM -0600, Bjorn Helgaas wrote:
> > > I think we have a problem with link bandwidth change notifications
> > > (see https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/pci/pcie/bw_notification.c).
> > >
> > > Here's a recent bug report where Jan reported "_tons_" of these
> > > notifications on an nvme device:
> > > https://bugzilla.kernel.org/show_bug.cgi?id=206197
> >
> > 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.
>
> Atanas is also seeing this problem and went to the trouble of digging
> up this bug report:
> https://bugzilla.kernel.org/show_bug.cgi?id=206197#c8
>
> I'm actually a little surprised that we haven't seen more reports of
> this.  I don't think distros enable CONFIG_PCIE_BW, but even so, I
> would think more people running upstream kernels would trip over it.
> But maybe people just haven't turned CONFIG_PCIE_BW on.
>
> I don't have a suggestion; just adding Atanas to this old thread.
>
> > > There was similar discussion involving GPU drivers at
> > > https://lore.kernel.org/r/20190429185611.121751-2-helgaas@xxxxxxxxxx
> > >
> > > The current solution is the CONFIG_PCIE_BW config option, which
> > > disables the messages completely.  That option defaults to "off" (no
> > > messages), but even so, I think it's a little problematic.
> > >
> > > Users are not really in a position to figure out whether it's safe to
> > > enable.  All they can do is experiment and see whether it works with
> > > their current mix of devices and drivers.
> > >
> > > I don't think it's currently useful for distros because it's a
> > > compile-time switch, and distros cannot predict what system configs
> > > will be used, so I don't think they can enable it.
> > >
> > > Does anybody have proposals for making it smarter about distinguishing
> > > real problems from intentional power management, or maybe interfaces
> > > drivers could use to tell us when we should ignore bandwidth changes?

There's also this recently filed bug:
https://gitlab.freedesktop.org/drm/amd/-/issues/1447
The root cause of it appears to be related to ASPM.

Alex