[PATCH AUTOSEL 6.6 17/40] selftests/bpf: fix bpf_loop_bench for new callback verification scheme

Sasha Levin
2023-11-28T21:07:44Z

From: Eduard Zingerman <eddyz87@gmail.com>

[ Upstream commit f40bfd1679446b22d321e64a1fa98b7d07d2be08 ]

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>
Link: https://lore.kernel.org/r/20231121020701.26440-4-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
tools/testing/selftests/bpf/progs/bpf_loop_bench.c | 13 ++++++++-----
1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/tools/testing/selftests/bpf/progs/bpf_loop_bench.c b/tools/testing/selftests/bpf/progs/bpf_loop_bench.c
index 4ce76eb064c41..d461746fd3c1e 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;
}
--
2.42.0