Re: [PATCH] PCI/LINK: Account for BW notification in vector calculation

Alex G
2019-04-23T16:03:09Z

On 4/23/19 10:34 AM, Alex Williamson wrote:
> On Tue, 23 Apr 2019 09:33:53 -0500
> Alex G <mr.nuke.me@gmail.com> 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@gmail.com> 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.

> 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.

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
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.

Alex

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