Heavy spin_lock contention in __udp4_lib_mcast_deliver increase

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I've been doing some UDP multicast benchmarking and noticed that as we
increase the number of sockets/multicast addresses the performance
degrades.  The test I'm running has multiple machines sending packets
on multiple multicast addresses.  A single receiving machine opens one
socket per multicast address to receive all the packets.  The
receiving process is bound to a core that is not processing
interrupts.

Running this test with 300 multicast addresses and sockets and
profiling the receiving machine with 'perf -a -g' I can see the
following:


# Events: 45K cycles
#
# Overhead
# ........  .....................................
#
    52.56%  [k] _raw_spin_lock
            |
            |--99.09%-- __udp4_lib_mcast_deliver
    20.10%  [k] __udp4_lib_mcast_deliver
            |
            --- __udp4_lib_rcv

So if I understand this correctly 52.56% of the time is spent
contending for the spin_lock in __udp4_lib_mcast_deliver.  If I
understand the code correctly it appears that for every packet
received we walk the list of all UDP sockets while holding the
spin_lock.  Therefore I believe the thing that hurts so much in this
case is that we have a lot of UDP sockets.

Are there any ideas on how we can improve the performance in this
case?  Honestly I have two ideas though my understanding of the
network stack is limited and it is unclear to me how to implement
either of them.

The first idea is to use RCU instead of acquiring the spin_lock.  This
is what the Unicast path does however looking back to 271b72c7 "udp:
RCU handling for Unicast packets." Eric points out that the multicast
path is difficult.  It appears from that commit description that the
problem is that since we have to find all sockets interested in
receiving the packet instead of just one that restarting the scan of
the hlist could lead us to deliver the packet twice to the same
socket.  That commit is rather old though I believe things may have
changed.  Looking at commit 1240d137 "ipv4: udp: Optimise multicast
reception" I can see that Eric also has already done some work to
reduce how long the spin_lock is held in __udp4_lib_mcast_deliver().
That commit also says "It's also a base for a future RCU conversion of
multicast recption".  Is the idea that you could remove duplicate
sockets within flush_stack()?  Actually I don't think that would work
since flush_stack() can be called multiple times if the stack gets
full.

The second idea would be to hash the sockets to reduce the number of
sockets to walk for each packet.  Once again it looks like the Unicast
path already does this in commits 512615b6b "udp: secondary hash on
(local port, local address)" and 5051ebd27 "ipv4: udp: optimize
unicast RX path".  Perhaps these hash lists could be used, however I
don't think they can since they currently use RCU and thus it might
depend on converting to RCU first.

Thanks,
Shawn

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