Re: Issues with "PCI/LINK: Report degraded links via link bandwidth notification"

From: Alex G <mr.nuke.me@gmail.com>
To: Bjorn Helgaas <helgaas@kernel.org>,
	Alexandru Gagniuc <alex_gagniuc@dellteam.com>,
	Keith Busch <keith.busch@intel.com>
Cc: Jan Vesely <jano.vesely@gmail.com>,
	Lukas Wunner <lukas@wunner.de>,
	Alex Williamson <alex.williamson@redhat.com>,
	Austin Bolen <austin_bolen@dell.com>,
	Shyam Iyer <Shyam_Iyer@dell.com>, Sinan Kaya <okaya@kernel.org>,
	linux-pci@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: Issues with "PCI/LINK: Report degraded links via link bandwidth notification"
Date: Wed, 15 Jan 2020 20:44:21 -0600	[thread overview]
Message-ID: <967fb44c-b1cd-875c-2354-b6ad0b8ae6d7@gmail.com> (raw)
In-Reply-To: <20200115221008.GA191037@google.com>

Hi Bjorn,

I'm no longer working on this, so my memory may not be up to speed. If 
the endpoint is causing the bandwidth change, then we should get an 
_autonomous_ link management interrupt instead. I don't think we report 
those, and that shouldn't spam the logs

If it's not a (non-autonomous) link management interrupt, then something 
is causing the downstream port to do funny things. I don't think ASPM is 
supposed to be causing this.

Do we know what's causing these swings?

For now, I suggest a boot-time parameter to disable link speed reporting 
instead of a compile time option.

Alex

On 1/15/20 4:10 PM, 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@kernel.org
> 
> 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?
> 
> Bjorn
> 