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

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Date:   Sat, 22 Feb 2020 10:58:40 -0600
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Alexandru Gagniuc <mr.nuke.me@...il.com>,
        Alexandru Gagniuc <alex_gagniuc@...lteam.com>,
        Keith Busch <keith.busch@...el.com>
Cc:     Jan Vesely <jano.vesely@...il.com>, Lukas Wunner <lukas@...ner.de>,
        Alex Williamson <alex.williamson@...hat.com>,
        Austin Bolen <austin_bolen@...l.com>,
        Shyam Iyer <Shyam_Iyer@...l.com>,
        Sinan Kaya <okaya@...nel.org>, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org, Christoph Hellwig <hch@....de>,
        Lucas Stach <l.stach@...gutronix.de>,
        Dave Airlie <airlied@...il.com>,
        Ben Skeggs <skeggsb@...il.com>,
        Alex Deucher <alexdeucher@...il.com>,
        Myron Stowe <myron.stowe@...hat.com>
Subject: Re: Issues with "PCI/LINK: Report degraded links via link bandwidth
 notification"

[+cc Christoph, Lucas, Dave, Ben, Alex, Myron]

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

AFAICT, this thread petered out with no resolution.

If the bandwidth change notifications are important to somebody,
please speak up, preferably with a patch that makes the notifications
disabled by default and adds a parameter to enable them (or some other
strategy that makes sense).

I think these are potentially useful, so I don't really want to just
revert them, but if nobody thinks these are important enough to fix,
that's a possibility.

> 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