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

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Date:   Tue, 23 Apr 2019 12:38:28 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Alex G <mr.nuke.me@...il.com>
Cc:     Bjorn Helgaas <helgaas@...nel.org>, linux-pci@...r.kernel.org,
        austin_bolen@...l.com, alex_gagniuc@...lteam.com,
        keith.busch@...el.com, Shyam_Iyer@...l.com, lukas@...ner.de,
        okaya@...nel.org, torvalds@...ux-foundation.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] PCI/LINK: Account for BW notification in vector
 calculation

On Tue, 23 Apr 2019 12:53:07 -0500
Alex G <mr.nuke.me@...il.com> wrote:

> On 4/23/19 12:10 PM, Bjorn Helgaas wrote:
> > On Tue, Apr 23, 2019 at 09:33:53AM -0500, Alex G wrote:  
> >> On 4/22/19 7:33 PM, Alex Williamson wrote:  
> >>> There is nothing wrong happening here that needs to fill logs.  I
> >>> thought maybe if I enabled notification of autonomous bandwidth
> >>> changes that it might categorize these as something we could
> >>> ignore, but it doesn't.  How can we identify only cases where this
> >>> is an erroneous/noteworthy situation?  Thanks,  
> >>
> >> You don't. Ethernet doesn't. USB doesn't. This logging behavior is
> >> consistent with every other subsystem that deals with multi-speed links.  
> > 
> > Can you point me to the logging in these other subsystems so I can
> > learn more about how they deal with this?  
> 
> I don't have any in-depth articles about the logging in these systems, 
> but I can extract some logs from my machines.
> 
> Ethernet:
> 
> [Sun Apr 21 11:14:06 2019] e1000e: eno1 NIC Link is Down
> [Sun Apr 21 11:14:17 2019] e1000e: eno1 NIC Link is Up 1000 Mbps Full 
> Duplex, Flow Control: Rx/Tx
> [Sun Apr 21 11:14:23 2019] e1000e: eno1 NIC Link is Up 1000 Mbps Full 
> Duplex, Flow Control: Rx/Tx
> [Sun Apr 21 23:33:31 2019] e1000e: eno1 NIC Link is Down
> [Sun Apr 21 23:33:43 2019] e1000e: eno1 NIC Link is Up 1000 Mbps Full 
> Duplex, Flow Control: Rx/Tx
> [Sun Apr 21 23:33:48 2019] e1000e: eno1 NIC Link is Up 1000 Mbps Full 
> Duplex, Flow Control: Rx/Tx
> 
> I used to have one of these "green" ethernet switches that went down to 
> 100mbps automatically. You can imagine how "clogged" the logs were with 
> link up messages. Thank goodness that switch was killed in a thunderstorm.
> 
> USB will log every device insertion and removal, very verbosely (see 
> appendix A).

I have a hard time putting USB insertion and removal into the same
class, the equivalent is PCI hotplug which is logged separately.  Do
we ever log beyond USB device discovery if a device is running at a
lower speed than is possible?  The most directly related is the green
ethernet switch, which you admit was a nuisance due to exactly this
sort of logging.  It was probably confusing to see this logging, perhaps
you wondered if the cable was bad or the switch was defective.

> > I agree that emitting log messages for normal and expected events will
> > lead to user confusion and we need to do something.
> > 
> > e8303bb7a75c ("PCI/LINK: Report degraded links via link bandwidth
> > notification") was merged in v5.1-rc1, so we still have (a little)
> > time to figure this out before v5.1.  
> 
> I always viewed the system log as a system log, instead of a database of 
> system errors. I may have extremist views, but going back to Alex's 
> example, I prefer to see that the power saving mechanism is doing 
> something to save power on my laptop (I'll just ignore it on a desktop).

There's a disconnect from above where similar behavior on ethernet
behavior "clogged" the log files, but here we just want to ignore it.
Excessive logging can also be considered a denial of service vector
when the device generating that excessive logging is attached to a
userspace driver.

> If you think increasing code complexity because people don't want things 
> logged into the system log, then I'm certain we can work out some sane 
> solution. It's the same problem we see with GCC, where people want 
> warning messages here, but don't want the same messages there.

v5.1 is approaching quickly, can we downgrade these to pci_dbg() while
we work on maybe some sort of driver participation in this logging?
Thanks,

Alex