netdev - [PATCH net-next 0/4] net: mitigate retpoline overhead

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Date:   Mon,  3 Dec 2018 12:40:20 +0100
From:   Paolo Abeni <pabeni@...hat.com>
To:     netdev@...r.kernel.org
Cc:     "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: [PATCH net-next 0/4] net: mitigate retpoline overhead

The spectre v2 counter-measures, aka retpolines, are a source of measurable
overhead[1]. We can partially address that when the function pointer refers to
a builtin symbol resorting to a list of tests vs well-known builtin function and
direct calls.

Experimental results[2] shows that replacing a single indirect calls via
retpoline with several branches and direct calls gives performance gains
even when multiple branches are added - 5 or more, as reported in [2].

This may lead to some uglification around the indirect calls. In netconf 2018
Eric Dumazet described a technique to hide the most relevant part of the needed
boilerplate with some macro help.

This series is a [re-]implementation of such idea, exposing the introduced
helpers in a new header file. They are later leveraged to avoid the indirect
call overhead in the GRO path, when possible.

Overall this gives > 10% performance improvement for UDP GRO benchmark, and
smaller but measurable for TCP syn flood.

The added infra can be used in follow-up patches to cope with retpoline overhead
in other points of the networking stack (e.g. at the qdisc layer) and possibly
even in other subsystems.

rfc -> v1:
 - use branch prediction hints, as suggested by Eric

[1] http://vger.kernel.org/netconf2018_files/PaoloAbeni_netconf2018.pdf
[2] https://linuxplumbersconf.org/event/2/contributions/99/attachments/98/117/lpc18_paper_af_xdp_perf-v2.pdf

Paolo Abeni (4):
  indirect call wrappers: helpers to speed-up indirect calls of builtin
  net: use indirect call wrappers at GRO network layer
  net: use indirect call wrapper at GRO transport layer
  udp: use indirect call wrapper for GRO socket lookup

 include/linux/indirect_call_wrapper.h | 77 +++++++++++++++++++++++++++
 include/net/inet_common.h             |  9 ++++
 net/core/dev.c                        | 10 +++-
 net/ipv4/af_inet.c                    | 15 +++++-
 net/ipv4/tcp_offload.c                |  5 ++
 net/ipv4/udp.c                        |  2 +
 net/ipv4/udp_offload.c                | 11 +++-
 net/ipv6/ip6_offload.c                | 14 ++++-
 net/ipv6/tcpv6_offload.c              |  5 ++
 net/ipv6/udp.c                        |  2 +
 net/ipv6/udp_offload.c                |  5 ++
 11 files changed, 147 insertions(+), 8 deletions(-)
 create mode 100644 include/linux/indirect_call_wrapper.h

-- 
2.19.2