Hi Everyone,
This is the next iteration of the patch. It includes changes suggested
by Song, Joanne and Alexei. 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 e.g. 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 v7:
- Call to `mark_chain_precision` is added in `loop_flag_is_zero` to
avoid potential issues with state pruning and precision tracking.
- `flags non-zero` test_verifier test case is updated to have two
execution paths reaching `bpf_loop` call, one with flags = 0,
another with flags = 1. Potentially this test case should be able
to show that call to `mark_chain_precision` is necessary in
`loop_flag_is_zero` but not at the moment. Please refer to
discussion for [PATCH bpf-next v7 3/5] for additional details.
- `stack_depth_extra` computation is updated to guarantee that R6, R7
and R8 offsets are always aligned on 8 byte boundary.
- `stack locations for loop vars` test_verifier test case updated to
show that R6, R7, R8 offsets are indeed aligned when function stack
depth is not a multiple of 8.
- I removed Song Liu's ACK from commit message for [PATCH bpf-next v8
4/5] because I updated the patch. (Please let me know if I had to
keep the ACK tag).
Changes since v6:
- Return value of the `optimize_bpf_loop` function is no longer
ignored. This is necessary to properly propagate -ENOMEM error.
Changes since v5:
- Added function `loop_flag_is_zero` to skip a few checks in
`update_loop_inline_state` when loop instruction is not fit for
inline.
Changes since v4:
- Added missing `static` modifier for `update_loop_inline_state` and
`inline_bpf_loop` functions.
- `update_loop_inline_state` updated for better readability.
- Fields `initialized` and `fit_for_inline` of `struct
bpf_loop_inline_state` are changed back from `bool` to bitfields.
- Acks from Song Liu added to comments for patches 1/5, 2/5, 4/5,
5/5.
Changes since v3:
- Function `adjust_stack_depth_for_loop_inlining` is replaced by
function `optimize_bpf_loop`. Function `optimize_bpf_loop` is
responsible for both stack depth adjustment and call instruction
replacement.
- Changes in `do_misc_fixups` are reverted.
- Changes in `adjust_subprog_starts_after_remove` are reverted and
function `adjust_loop_inline_subprogno` is removed. This is
possible because call to `optimize_bpf_loop` is placed before the
dead code removal in `opt_remove_dead_code` (in contrast to the
position of `do_misc_fixups` where inlining was done in v3).
- Field `bpf_insn_aux_data.loop_inline_state` is now a part of
anonymous union at the start of the `bpf_insn_aux_data`.
- Data structure `bpf_loop_inline_state` is simplified to use single
flag field `fit_for_inline` instead of separate fields
`flags_is_zero` & `callback_is_constant`.
- Macro definition `BPF_MAX_LOOPS` is moved from
`include/linux/bpf_verifier.h` to `include/linux/bpf.h` to avoid
include of `include/linux/bpf_verifier.h` in `bpf_iter.c`.
- `inline_bpf_loop` changed back to use array initialization and hard
coded offsets as in v2.
- Style / formatting updates.
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.h | 3 +
include/linux/bpf_verifier.h | 12 +
kernel/bpf/bpf_iter.c | 9 +-
kernel/bpf/verifier.c | 180 +++++++++-
.../selftests/bpf/prog_tests/bpf_loop.c | 62 ++++
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 | 328 +++++++++++++++++-
.../selftests/bpf/verifier/bpf_loop_inline.c | 252 ++++++++++++++
10 files changed, 936 insertions(+), 27 deletions(-)
create mode 100644 tools/testing/selftests/bpf/verifier/bpf_loop_inline.c
--
2.25.1
