diff mbox series

[bpf,v3,03/11] selftests/bpf: fix bpf_loop_bench for new callback verification scheme

Message ID 20231120225945.11741-4-eddyz87@gmail.com (mailing list archive)
State Superseded
Delegated to: BPF
Headers show
Series verify callbacks as if they are called unknown number of times | expand

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bpf/vmtest-bpf-PR fail PR summary
bpf/vmtest-bpf-VM_Test-30 fail Logs for x86_64-llvm-16 / test (test_progs, false, 360) / test_progs on x86_64 with llvm-16
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bpf/vmtest-bpf-VM_Test-32 success Logs for x86_64-llvm-16 / test (test_verifier, false, 360) / test_verifier on x86_64 with llvm-16
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bpf/vmtest-bpf-VM_Test-20 success Logs for x86_64-gcc / test (test_progs_no_alu32_parallel, true, 30) / test_progs_no_alu32_parallel on x86_64 with gcc
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bpf/vmtest-bpf-VM_Test-24 success Logs for x86_64-llvm-16 / build / build for x86_64 with llvm-16
bpf/vmtest-bpf-VM_Test-25 success Logs for x86_64-llvm-16 / test (test_maps, false, 360) / test_maps on x86_64 with llvm-16
bpf/vmtest-bpf-VM_Test-26 success Logs for x86_64-llvm-16 / test (test_progs, false, 360) / test_progs on x86_64 with llvm-16
bpf/vmtest-bpf-VM_Test-27 success Logs for x86_64-llvm-16 / test (test_progs_no_alu32, false, 360) / test_progs_no_alu32 on x86_64 with llvm-16
bpf/vmtest-bpf-VM_Test-28 success Logs for x86_64-llvm-16 / test (test_verifier, false, 360) / test_verifier on x86_64 with llvm-16
bpf/vmtest-bpf-VM_Test-29 success Logs for x86_64-llvm-16 / veristat
bpf/vmtest-bpf-VM_Test-13 success Logs for s390x-gcc / test (test_verifier, false, 360) / test_verifier on s390x with gcc
bpf/vmtest-bpf-VM_Test-11 success Logs for s390x-gcc / test (test_progs, false, 360) / test_progs on s390x with gcc
bpf/vmtest-bpf-VM_Test-12 success Logs for s390x-gcc / test (test_progs_no_alu32, false, 360) / test_progs_no_alu32 on s390x with gcc
bpf/vmtest-bpf-VM_Test-10 success Logs for s390x-gcc / test (test_maps, false, 360) / test_maps on s390x with gcc
netdev/series_format success Posting correctly formatted
netdev/codegen success Generated files up to date
netdev/tree_selection success Clearly marked for bpf, async
netdev/fixes_present fail Series targets non-next tree, but doesn't contain any Fixes tags
netdev/header_inline success No static functions without inline keyword in header files
netdev/build_32bit success Errors and warnings before: 8 this patch: 8
netdev/cc_maintainers warning 9 maintainers not CCed: mykolal@fb.com song@kernel.org haoluo@google.com linux-kselftest@vger.kernel.org jolsa@kernel.org kpsingh@kernel.org john.fastabend@gmail.com shuah@kernel.org sdf@google.com
netdev/build_clang success Errors and warnings before: 8 this patch: 8
netdev/verify_signedoff success Signed-off-by tag matches author and committer
netdev/deprecated_api success None detected
netdev/check_selftest success No net selftest shell script
netdev/verify_fixes success No Fixes tag
netdev/build_allmodconfig_warn success Errors and warnings before: 8 this patch: 8
netdev/checkpatch success total: 0 errors, 0 warnings, 0 checks, 21 lines checked
netdev/build_clang_rust success No Rust files in patch. Skipping build
netdev/kdoc success Errors and warnings before: 0 this patch: 0
netdev/source_inline success Was 0 now: 0

Commit Message

Eduard Zingerman Nov. 20, 2023, 10:59 p.m. UTC
This is a preparatory change. A follow-up patch "bpf: verify callbacks
as if they are called unknown number of times" changes logic for
callbacks handling. While previously callbacks were verified as a
single function call, new scheme takes into account that callbacks
could be executed unknown number of times.

This has dire implications for bpf_loop_bench:

    SEC("fentry/" SYS_PREFIX "sys_getpgid")
    int benchmark(void *ctx)
    {
            for (int i = 0; i < 1000; i++) {
                    bpf_loop(nr_loops, empty_callback, NULL, 0);
                    __sync_add_and_fetch(&hits, nr_loops);
            }
            return 0;
    }

W/o callbacks change verifier sees it as a 1000 calls to
empty_callback(). However, with callbacks change things become
exponential:
- i=0: state exploring empty_callback is scheduled with i=0 (a);
- i=1: state exploring empty_callback is scheduled with i=1;
  ...
- i=999: state exploring empty_callback is scheduled with i=999;
- state (a) is popped from stack;
- i=1: state exploring empty_callback is scheduled with i=1;
  ...

Avoid this issue by rewriting outer loop as bpf_loop().
Unfortunately, this adds a function call to a loop at runtime, which
negatively affects performance:

            throughput               latency
   before:  149.919 ± 0.168 M ops/s, 6.670 ns/op
   after :  137.040 ± 0.187 M ops/s, 7.297 ns/op

Acked-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
---
 tools/testing/selftests/bpf/progs/bpf_loop_bench.c | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)
diff mbox series

Patch

diff --git a/tools/testing/selftests/bpf/progs/bpf_loop_bench.c b/tools/testing/selftests/bpf/progs/bpf_loop_bench.c
index 4ce76eb064c4..d461746fd3c1 100644
--- a/tools/testing/selftests/bpf/progs/bpf_loop_bench.c
+++ b/tools/testing/selftests/bpf/progs/bpf_loop_bench.c
@@ -15,13 +15,16 @@  static int empty_callback(__u32 index, void *data)
 	return 0;
 }
 
+static int outer_loop(__u32 index, void *data)
+{
+	bpf_loop(nr_loops, empty_callback, NULL, 0);
+	__sync_add_and_fetch(&hits, nr_loops);
+	return 0;
+}
+
 SEC("fentry/" SYS_PREFIX "sys_getpgid")
 int benchmark(void *ctx)
 {
-	for (int i = 0; i < 1000; i++) {
-		bpf_loop(nr_loops, empty_callback, NULL, 0);
-
-		__sync_add_and_fetch(&hits, nr_loops);
-	}
+	bpf_loop(1000, outer_loop, NULL, 0);
 	return 0;
 }