On Tue, 23 Apr 2019 11:03:04 -0500 Alex G <mr.nuke.me@xxxxxxxxx> wrote: > On 4/23/19 10:34 AM, Alex Williamson wrote: > > On Tue, 23 Apr 2019 09:33:53 -0500 > > Alex G <mr.nuke.me@xxxxxxxxx> wrote: > > > >> On 4/22/19 7:33 PM, Alex Williamson wrote: > >>> On Mon, 22 Apr 2019 19:05:57 -0500 > >>> Alex G <mr.nuke.me@xxxxxxxxx> wrote: > >>>> echo 0000:07:00.0:pcie010 | > >>>> sudo tee /sys/bus/pci_express/drivers/pcie_bw_notification/unbind > >>> > >>> That's a bad solution for users, this is meaningless tracking of a > >>> device whose driver is actively managing the link bandwidth for power > >>> purposes. > >> > >> 0.5W savings on a 100+W GPU? I agree it's meaningless. > > > > Evidence? Regardless, I don't have control of the driver that's making > > these changes, but the claim seems unfounded and irrelevant. > > The number of 5mW/Gb/lane doesn't ring a bell? [1] [2]. Your GPU > supports 5Gb/s, so likely using an older, more power hungry process. I > suspect it's still within the same order of magnitude. This doesn't necessarily imply the overall power savings to the endpoint as a whole though, and it's still irrelevant to the discussion here. The driver is doing something reasonable that's generating host dmesg spam. > > I'm assigning a device to a VM [snip] > > I can see why we might want to be notified of degraded links due to signal issues, > > but what I'm reporting is that there are also entirely normal reasons > > [snip] we can't seem to tell the difference > > Unfortunately, there is no way in PCI-Express to distinguish between an > expected link bandwidth change and one due to error. Then assuming every link speed change is an error seems like the wrong approach. Should we instead have a callback that drivers can optionally register to receive link change notifications? If a driver doesn't register such a callback then a generic message can be posted, but if they do, the driver can decide whether this is an error. > If you're using virt-manager to configure the VM, then virt-manager > could have a checkbox to disable link bandwidth management messages. I'd What makes us think that this is the only case where such link speed changes will occur? Hand waving that a userspace management utility should go unbind drivers that over-zealously report errors is a poor solution. > rather we avoid kernel-side heuristics (like Lukas suggested). If you're > confident that your link will operate as intended, and don't want > messages about it, that's your call as a user -- we shouldn't decide > this in the kernel. Nor should pci-core decide what link speed changes are intended or errors. Minimally we should be enabling drivers to receive this feedback. Thanks, Alex
