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

[Alex G]

Hi Bjorn,

I'm no longer working on this, so my memory may not be up to speed. If 
the endpoint is causing the bandwidth change, then we should get an 
_autonomous_ link management interrupt instead. I don't think we report 
those, and that shouldn't spam the logs

If it's not a (non-autonomous) link management interrupt, then something 
is causing the downstream port to do funny things. I don't think ASPM is 
supposed to be causing this.

Do we know what's causing these swings?

For now, I suggest a boot-time parameter to disable link speed reporting 
instead of a compile time option.

Alex

On 1/15/20 4:10 PM, 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?
>
> Bjorn