Bug#820622: linux-image-4.5.0-trunk-armmp-lpae: raspberry pi 2: smsc95xx 1-1.1:1.0 eth0: kevent 2 may have been dropped

[Bjørn Mork <bjorn@mork.no>]

Ben Hutchings <ben@decadent.org.uk> writes:
> On Sun, 2016-04-10 at 11:15 -0700, Vagrant Cascadian wrote:
>> It works for the most part, but floods syslog with messages:
>> 
>> [501966.870273] net_ratelimit: 35702 callbacks suppressed
>> [501966.875438] smsc95xx 1-1.1:1.0 eth0: kevent 2 may have been dropped
>> 
>> It seems to function ok, though I'm not sure if there's degraded
>> network performance...
> [...]
>
> I understand network performance on all RPi models is poor due to lack
> of a built-in Ethernet MAC and the poor design of the USB interface.
> Also that message is a generic error message from the usbnet core and
> is not specific to the smsc95xx driver.

Yes, kevent 2 is "EVENT_RX_MEMORY", and this error affects any usbnet
based device under memory pressure.  The usbnet framework just doesn't
handle the case where GFP_KERNEL allocations fail. How could it? 

The only viable "fix" I can see is by preallocating and recycling all
buffers instead of the repeated allocations done by usbnet now.  But
that's a major refactoring of usbnet.

The log message spam could of course be fixed, but the rate-limiting is
"good enough".


Bjørn