On Tue, 21 Jan 2020 at 21:11, Lucas Stach <l.stach@xxxxxxxxxxxxxx> 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. > Adding Alex and Ben for AMD and NVIDIA info Dave.
