netdev - Re: UDP sockets oddities

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Message-ID: <1503712322.11498.12.camel@edumazet-glaptop3.roam.corp.google.com>
Date:   Fri, 25 Aug 2017 18:52:02 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Florian Fainelli <f.fainelli@...il.com>
Cc:     netdev@...r.kernel.org, edumazet@...il.com, pabeni@...hat.com,
        willemb@...gle.com, davem@...emloft.net
Subject: Re: UDP sockets oddities

On Fri, 2017-08-25 at 18:17 -0700, Florian Fainelli wrote:
> On 08/25/2017 04:57 PM, Eric Dumazet wrote:
> > On Fri, 2017-08-25 at 16:18 -0700, Florian Fainelli wrote:
> > 
> >> Eric, are there areas of the stack where we are allowed to drop packets,
> >> not propagate that back to write(2) and also not increment any counter
> >> either, or maybe I am not looking where I should...
> > 
> > What happens if you increase these sysctls ?
> 
> I don't see packet loss after I tweak these two sysctls according to
> your suggestions.
> 
> Tweaking eth0's sysctls did not change anything, but tweaking gphy's
> sysctl resolved the loss. This was a little surprising considering that
> gphy is an IFF_NO_QUEUE interface and eth0 is the conduit interface that
> does the real transmission.
> 
> Does that make sense with respect to what I reported earlier? Should I
> try to dump the neigh stats?

Note that if you had TCP traffic, the neighbour would be constantly
confirmed and no losses would happen.

I guess we should an SNMP counter for packets dropped in neigh queues.