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

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Date:	Tue, 26 Oct 2010 16:21:16 -0400
From:	"Jeremy Jackson" <jerj@...lanar.net>
To:	"Eric Dumazet" <eric.dumazet@...il.com>
Cc:	"Jeremy Jackson" <jerj@...lanar.net>, netdev@...r.kernel.org
Subject: Re: Unwanted aliasing of UDP checksum failed error counter

> 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.

> udpInErrors OBJECT-TYPE
>     SYNTAX     Counter32
>     MAX-ACCESS read-only
>     STATUS     current
>     DESCRIPTION
>            "The number of received UDP datagrams that could not be
>             delivered for reasons other than the lack of an
>             application at the destination port.
>
>
> We could add a LINUX specific MIB entry, eventually...
>
>
>
>


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