From: Alex Williamson <alex.williamson@redhat.com>
To: Alex G <mr.nuke.me@gmail.com>
Cc: bhelgaas@google.com, helgaas@kernel.org,
linux-pci@vger.kernel.org, austin_bolen@dell.com,
alex_gagniuc@dellteam.com, keith.busch@intel.com,
Shyam_Iyer@Dell.com, lukas@wunner.de, okaya@kernel.org,
torvalds@linux-foundation.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] PCI/LINK: Account for BW notification in vector calculation
Date: Tue, 23 Apr 2019 10:22:53 -0600 [thread overview]
Message-ID: <20190423102253.4fd9a019@x1.home> (raw)
In-Reply-To: <88bd2962-bce4-8259-c38f-1a7e9fdde300@gmail.com>
On Tue, 23 Apr 2019 11:03:04 -0500
Alex G <mr.nuke.me@gmail.com> 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@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.
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
next prev parent reply other threads:[~2019-04-23 16:22 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-04-22 22:43 [PATCH] PCI/LINK: Account for BW notification in vector calculation Alex Williamson
2019-04-23 0:05 ` Alex G
2019-04-23 0:33 ` Alex Williamson
2019-04-23 14:33 ` Alex G
2019-04-23 15:34 ` Alex Williamson
2019-04-23 15:49 ` Lukas Wunner
2019-04-23 16:03 ` Alex G
2019-04-23 16:22 ` Alex Williamson [this message]
2019-04-23 16:27 ` Alex G
2019-04-23 16:37 ` Alex Williamson
2019-04-23 17:10 ` Bjorn Helgaas
2019-04-23 17:53 ` Alex G
2019-04-23 18:38 ` Alex Williamson
2019-04-23 17:59 ` Alex G
2019-05-01 20:30 ` Bjorn Helgaas
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