[RFC PATCH 0/2] udp: avoid false sharing on sk_tsflags

[Paolo Abeni <pabeni@redhat.com>]

While benchmarking the recently shared page frag revert, I observed a
lot of cache misses in the UDP RX path due to false sharing between the
sk_tsflags and the sk_forward_alloc sk fields.

Here comes a solution attempt for such a problem, inspired by commit
f796feabb9f5 ("udp: add local "peek offset enabled" flag").

The first patch adds a new proto op allowing protocol specific operation
on tsflags updates, and the 2nd one leverages such operation to cache
the problematic field in a cache friendly manner.

The need for a new operation is possibly suboptimal, hence the RFC tag,
but I could not find other good solutions. I considered:
- moving the sk_tsflags just before 'sk_policy', in the 'sock_read_rxtx'
  group. It arguably belongs to such group, but the change would create
  a couple of holes, increasing the 'struct sock' size and would have 
  side effects on other protocols
- moving the sk_tsflags just before 'sk_stamp'; similar to the above,
  would possibly reduce the side effects, as most of 'struct sock'
  layout will be unchanged. Could increase the number of cacheline
  accessed in the TX path.

I opted for the present solution as it should minimize the side effects
to other protocols.

Paolo Abeni (2):
  sock: introduce set_tsflags operation
  udp: avoid false sharing via protocol specific set_tsflags

 include/linux/udp.h | 12 ++++++++++++
 include/net/sock.h  | 15 +++++++++++----
 include/net/tcp.h   |  1 +
 net/core/sock.c     | 24 +++++++++---------------
 net/ipv4/tcp.c      | 16 ++++++++++++++++
 net/ipv4/tcp_ipv4.c |  1 +
 net/ipv4/udp.c      |  3 ++-
 net/ipv6/tcp_ipv6.c |  1 +
 net/ipv6/udp.c      |  3 ++-
 9 files changed, 55 insertions(+), 21 deletions(-)

-- 
2.48.1