netdev - Re: Unwanted aliasing of UDP checksum failed error counter

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Date:	Tue, 26 Oct 2010 22:27:22 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jeremy Jackson <jerj@...lanar.net>
Cc:	netdev@...r.kernel.org
Subject: Re: Unwanted aliasing of UDP checksum failed error counter

Le mardi 26 octobre 2010 à 16:21 -0400, Jeremy Jackson a écrit :
> > Le mardi 26 octobre 2010 Ã  15:53 -0400, Jeremy Jackson a Ã©crit :
> >> Trying to find source of packet loss on an 8node compute cluster, we
> >> find:
> >> (not in this example, but on the real cluster)
> >>
> >> in /proc/sys/net/snmp
> >> Udp: InDatagrams NoPorts InErrors OutDatagrams RcvbufErrors SndbufErrors
> >> Udp: 976460 1750 0 986795 0 0
> >>
> >> InErrors *and* RcvbufErrors both go up with full socket buffer, this has
> >> made troubleshooting our application more difficult.  We were chasing
> >> UDP
> >> checksum problems, until we checked linux source code, and found
> >> aliasing.
> >>
> >> Is this done for assembly code efficiency?  Any reason ENOMEM (ie socket
> >> buffer full) can't avoid aliasing to UDP checksum failed errors?
> >>
> >> in linux-source-2.6.32/net/ipv4/udp.c:__udp_queue_rcv_skb()
> >> ....
> >>                 /* Note that an ENOMEM error is charged twice */
> >>                 if (rc == -ENOMEM) {
> >>                         UDP_INC_STATS_BH(sock_net(sk),
> >> UDP_MIB_RCVBUFERRORS,
> >>                                          is_udplite);
> >>                         atomic_inc(&sk->sk_drops);
> >>                 }
> >>                 goto drop;
> >> ...
> >> drop:
> >>         UDP_INC_STATS_BH(sock_net(sk), UDP_MIB_INERRORS, is_udplite);
> >>
> >
> > In MIBS, there is no counter for UDP checksum errors
> >
> > So we use the standard UDP_MIB_INERRORS
> 
> Yes, this part I understand, but what I don't understand is why ENOMEM
> errors *and* checksum errors both use the same counter, while ENOMEM has
> it's own already.

Because ENOMEM errors were handled in commit 81aa646c,
but _all_ errors must also be accounted in INERRORS, to be RFC
compliant.

[IPV4]: add the UdpSndbufErrors and UdpRcvbufErrors MIBs


If we add a new MIB counter for checksum errors, a bad checksum packet
will increment both this new counter and INERRORS.



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