On Sun, 19 Jan 2020 20:33:26 -0600 Bjorn Helgaas <helgaas@xxxxxxxxxx> 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? One case where we previously saw sporadic link change messages was vfio-pci owned devices. If the transitions are based on config space manipulation then I can trap those accesses and wrap them in a PCI core API, but I suspect that's not the exclusive (or potentially even primary) mechanism for initiating link changes. So I think we'd probably need a mechanism for a driver to opt-out of link notification for their devices (presumably the fn0 device per link would opt-out the entire link?). Thanks, Alex
