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

From: Alex G
Date: Wed Jan 15 2020 - 21:44:27 EST


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