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

[Alex G.]

On 1/21/20 5:10 AM, Lucas Stach wrote:
> On Mo, 2020-01-20 at 10:01 -0600, Alex G. wrote:
> > On 1/19/20 8:33 PM, Bjorn Helgaas wrote:
> > > [+cc NVMe, GPU driver folks]
> > >
> > > 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
> > > >
> > > > 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?
> > >
> > > NVMe, GPU folks, do your drivers or devices change PCIe link
> > > speed/width for power saving or other reasons?  When CONFIG_PCIE_BW=y,
> > > the PCI core interprets changes like that as problems that need to be
> > > reported.
> > >
> > > If drivers do change link speed/width, can you point me to where
> > > that's done?  Would it be feasible to add some sort of PCI core
> > > interface so the driver could say "ignore" or "pay attention to"
> > > subsequent link changes?
> > >
> > > Or maybe there would even be a way to move the link change itself into
> > > the PCI core, so the core would be aware of what's going on?
> >
> > Funny thing is, I was going to suggest an in-kernel API for this.
> >     * Driver requests lower link speed 'X'
> >     * Link management interrupt fires
> >     * If link speed is at or above 'X' then do not report it.
> > I think an "ignore" flag would defeat the purpose of having link
> > bandwidth reporting in the first place. If some drivers set it, and
> > others don't, then it would be inconsistent enough to not be useful.
> >
> > A second suggestion is, if there is a way to ratelimit these messages on
> > a per-downstream port basis.
>
> Both AMD and Nvidia GPUs have embedded controllers, which are
> responsible for the power management. IIRC those controllers can
> autonomously initiate PCIe link speed changes depending on measured bus
> load. So there is no way for the driver to signal the requested bus
> speed to the PCIe core.
>
> I guess for the GPU usecase the best we can do is to have the driver
> opt-out of the link bandwidth notifications, as the driver knows that
> there is some autonomous entity on the endpoint mucking with the link
> parameters.

I'm confused. Are you saying that the autonomous mechanism is causing a 
link bandwidth notification?

Alex