Re: [PATCH net-next] tcp: export drops counter to /proc/net/tcp{,6}

On Fri, 2017-08-18 at 03:21 -0700, Stéphan Gorget wrote:

Those counters are exported for raw and udp but not for tcp, though they
are incremented.

An example where it is useful is chasing listen overflow. Listen overflow
are counted as a global counter in LINUX_MIB_LISTENOVERFLOWS accessible
in /proc/net/netstat but there is no way to find related drops in the
information exported for tcp. With this patch it will make possible to
correlate growth of LINUX_MIB_LISTENOVERFLOWS with growth of drops for
a tcp socket.

Hi Stéphan.

Eons ago, the decision was taken to declare /proc interface frozen.

In linux-4.7 I provided all the information you need using the modern
inet_diag interface, [1]

Simply use iproute2/ss tool to access this information in a very
efficient way (like filtering done in the kernel, instead having to
parse a gigantic /proc output)

lpaa5:~# ss -tm state listening src :22
Recv-Q Send-Q Local Address:Port                 Peer Address:Port                
0      128             *:ssh                           *:*                    
	 skmem:(r0,rb8388608,t0,tb8388608,f0,w0,o0,bl0,d7)
0      128            :::ssh                          :::*                    
	 skmem:(r0,rb8388608,t0,tb8388608,f0,w0,o0,bl0,d0)

You can see here that the IPv4 listener for ssh had 7 drops.

Thanks.

[1] list of relevant commits.

commit 9caad864151e525929d323de96cad382da49c3b2
Author: Eric Dumazet [off-list ref]
Date:   Fri Apr 1 08:52:20 2016 -0700

    tcp: increment sk_drops for listeners
    
    Goal: packets dropped by a listener are accounted for.
    
    This adds tcp_listendrop() helper, and clears sk_drops in sk_clone_lock()
    so that children do not inherit their parent drop count.
    
    Note that we no longer increment LINUX_MIB_LISTENDROPS counter when
    sending a SYNCOOKIE, since the SYN packet generated a SYNACK.
    We already have a separate LINUX_MIB_SYNCOOKIESSENT
    
    Signed-off-by: Eric Dumazet [off-list ref]
    Signed-off-by: David S. Miller [off-list ref]

commit 532182cd610782db8c18230c2747626562032205
Author: Eric Dumazet [off-list ref]
Date:   Fri Apr 1 08:52:19 2016 -0700

    tcp: increment sk_drops for dropped rx packets
    
    Now ss can report sk_drops, we can instruct TCP to increment
    this per socket counter when it drops an incoming frame, to refine
    monitoring and debugging.
    
    Following patch takes care of listeners drops.
    
    Signed-off-by: Eric Dumazet [off-list ref]
    Signed-off-by: David S. Miller [off-list ref]

commit 15239302edd46b184e758048253541fb211e315e
Author: Eric Dumazet [off-list ref]
Date:   Fri Apr 1 08:52:18 2016 -0700

    sock_diag: add SK_MEMINFO_DROPS
    
    Reporting sk_drops to user space was available for UDP
    sockets using /proc interface.
    
    Add this to sock_diag, so that we can have the same information
    available to ss users, and we'll be able to add sk_drops
    indications for TCP sockets as well.
    
    Signed-off-by: Eric Dumazet [off-list ref]
    Signed-off-by: David S. Miller [off-list ref]