[bpf-next,v3,0/5] bpf_loop inlining

Message

Eduard Zingerman June 3, 2022, 2:10 p.m. UTC

Hi Everyone,

This is the next iteration of the patch. Please find updated intro
message and change log below.

This patch implements inlining of calls to bpf_loop helper function
when bpf_loop's callback is statically known. E.g. the rewrite does
the following transformation during BPF program processing:

  bpf_loop(10, foo, NULL, 0);

 ->

  for (int i = 0; i < 10; ++i)
    foo(i, NULL);

The transformation leads to measurable latency change for simple
loops. Measurements using `benchs/run_bench_bpf_loop.sh` inside QEMU /
KVM on i7-4710HQ CPU show a drop in latency from 14 ns/op to 2 ns/op.

The change is split in five parts:

* Update to test_verifier.c to specify expected and unexpected
  instruction sequences. This allows to check BPF program rewrites
  applied by do_mix_fixups function.

* Update to test_verifier.c to specify BTF function infos and types
  per test case. This is necessary for tests that load sub-program
  addresses to a variable because of the checks applied by
  check_ld_imm function.

* The update to verifier.c that tracks state of the parameters for
  each bpf_loop call in a program and decides whether it could be
  replaced by a loop.

* A set of test cases for `test_verifier` that use capabilities added
  by the first two patches to verify instructions produced by inlining
  logic.

* Two test cases for `test_prog` to check that possible corner cases
  behave as expected.

Additional details are available in commit messages for each patch.

Changes since v2:
 - fix for `stack_check` test case in `test_progs-no_alu32`, all tests
   are passing now;
 - v2 3/3 patch is split in three parts:
   - kernel changes
   - test_verifier changes
   - test_prog changes
 - updated `inline_bpf_loop` in `verifier.c` to calculate each offset
   used in instructions to avoid "magic" numbers;
 - removed newline handling logic in `fail_log` branch of
   `do_single_test` in `test_verifier.c` to simplify the patch set;
 - styling fixes suggested in review for v2 of this patch set.

Changes since v1:
 - allow to use SKIP_INSNS in instruction pattern specification in
   test_verifier tests;
 - fix for a bug in spill offset assignement for loop vars when
   bpf_loop is located in a non-main function.

Eduard Zingerman (5):
  selftests/bpf: specify expected instructions in test_verifier tests
  selftests/bpf: allow BTF specs and func infos in test_verifier tests
  bpf: Inline calls to bpf_loop when callback is known
  selftests/bpf: BPF test_verifier selftests for bpf_loop inlining
  selftests/bpf: BPF test_prog selftests for bpf_loop inlining

 include/linux/bpf_verifier.h                  |  16 +
 kernel/bpf/bpf_iter.c                         |   9 +-
 kernel/bpf/verifier.c                         | 199 ++++++++++-
 .../selftests/bpf/prog_tests/bpf_loop.c       |  61 ++++
 tools/testing/selftests/bpf/prog_tests/btf.c  |   1 -
 tools/testing/selftests/bpf/progs/bpf_loop.c  | 114 +++++++
 tools/testing/selftests/bpf/test_btf.h        |   2 +
 tools/testing/selftests/bpf/test_verifier.c   | 316 +++++++++++++++++-
 .../selftests/bpf/verifier/bpf_loop_inline.c  | 244 ++++++++++++++
 9 files changed, 935 insertions(+), 27 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/verifier/bpf_loop_inline.c