[PATCH RFC net-next 0/3] net/sched: retpoline wrappers for tc

[Pedro Tammela <pctammela@gmail.com>]

In tc all qdics, classifiers and actions can be compiled as modules.
This results today in indirect calls in all transitions in the tc hierarchy.
Due to CONFIG_RETPOLINE, CPUs with mitigations=on might pay an extra cost on
indirect calls. For newer Intel cpus with IBRS the extra cost is
nonexistent, but AMD Zen cpus and older x86 cpus still go through the
retpoline thunk.

Known built-in symbols can be optimized into direct calls, thus
avoiding the retpoline thunk. So far, tc has not been leveraging this
build information and leaving out a performance optimization for some
CPUs. In this series we wire up 'tcf_classify()' and 'tcf_action_exec()'
with direct calls when known modules are compiled as built-in as an
opt-in optimization.

We measured these changes in one AMD Zen 3 cpu (Retpoline), one Intel 10th
Gen CPU (IBRS), one Intel 3rd Gen cpu (Retpoline) and one Intel Xeon CPU (IBRS)
using pktgen with 64b udp packets. Our test setup is a dummy device with
clsact and matchall in a kernel compiled with every tc module as built-in.
We observed a 6-10% speed up on the retpoline CPUs, when going through 1
tc filter, and a 60-100% speed up when going through 100 filters.
For the IBRS cpus we observed a 1-2% degradation in both scenarios, we believe
the extra branches checks introduced a small overhead therefore we added
a Kconfig option to make these changes opt-in even in CONFIG_RETPOLINE kernels.

We are continuing to test on other hardware variants as we find them:

1 filter:
CPU        | before (pps) | after (pps) | diff
R9 5950X   | 4237838      | 4412241     | +4.1%
R9 5950X   | 4265287      | 4413757     | +3.4%   [*]
i5-3337U   | 1580565      | 1682406     | +6.4%
i5-10210U  | 3006074      | 3006857     | +0.0%
i5-10210U  | 3160245      | 3179945     | +0.6%   [*]
Xeon 6230R | 3196906      | 3197059     | +0.0%
Xeon 6230R | 3190392      | 3196153     | +0.01%  [*]

100 filters:
CPU        | before (pps) | after (pps) | diff
R9 5950X   | 313469       | 633303      | +102.03%
R9 5950X   | 313797       | 633150      | +101.77% [*]
i5-3337U   | 127454       | 211210      | +65.71%
i5-10210U  | 389259       | 381765      | -1.9%
i5-10210U  | 408812       | 412730      | +0.9%    [*]
Xeon 6230R | 415420       | 406612      | -2.1%
Xeon 6230R | 416705       | 405869      | -2.6%    [*]

[*] In these tests we ran pktgen with clone set to 1000.

Pedro Tammela (3):
  net/sched: add retpoline wrapper for tc
  net/sched: avoid indirect act functions on retpoline kernels
  net/sched: avoid indirect classify functions on retpoline kernels

 include/net/tc_wrapper.h   | 274 +++++++++++++++++++++++++++++++++++++
 net/sched/Kconfig          |  13 ++
 net/sched/act_api.c        |   3 +-
 net/sched/act_bpf.c        |   6 +-
 net/sched/act_connmark.c   |   6 +-
 net/sched/act_csum.c       |   6 +-
 net/sched/act_ct.c         |   4 +-
 net/sched/act_ctinfo.c     |   6 +-
 net/sched/act_gact.c       |   6 +-
 net/sched/act_gate.c       |   6 +-
 net/sched/act_ife.c        |   6 +-
 net/sched/act_ipt.c        |   6 +-
 net/sched/act_mirred.c     |   6 +-
 net/sched/act_mpls.c       |   6 +-
 net/sched/act_nat.c        |   7 +-
 net/sched/act_pedit.c      |   6 +-
 net/sched/act_police.c     |   6 +-
 net/sched/act_sample.c     |   6 +-
 net/sched/act_simple.c     |   6 +-
 net/sched/act_skbedit.c    |   6 +-
 net/sched/act_skbmod.c     |   6 +-
 net/sched/act_tunnel_key.c |   6 +-
 net/sched/act_vlan.c       |   6 +-
 net/sched/cls_api.c        |   3 +-
 net/sched/cls_basic.c      |   6 +-
 net/sched/cls_bpf.c        |   6 +-
 net/sched/cls_cgroup.c     |   6 +-
 net/sched/cls_flow.c       |   6 +-
 net/sched/cls_flower.c     |   6 +-
 net/sched/cls_fw.c         |   6 +-
 net/sched/cls_matchall.c   |   6 +-
 net/sched/cls_route.c      |   6 +-
 net/sched/cls_rsvp.c       |   2 +
 net/sched/cls_rsvp.h       |   7 +-
 net/sched/cls_rsvp6.c      |   2 +
 net/sched/cls_tcindex.c    |   7 +-
 net/sched/cls_u32.c        |   6 +-
 37 files changed, 417 insertions(+), 67 deletions(-)
 create mode 100644 include/net/tc_wrapper.h

-- 
2.34.1