diff options
| author | Matt Bobrowski <mattbobrowski@google.com> | 2025-11-19 14:35:40 +0000 |
|---|---|---|
| committer | Alexei Starovoitov <ast@kernel.org> | 2025-11-21 16:49:16 -0800 |
| commit | ae24fc8a16b0481ea8c5acbc66453c49ec0431c4 (patch) | |
| tree | 78e6746e4077a332dcb234439fad775d063025cd /tools/testing/selftests/bpf/progs/test_perf_branches.c | |
| parent | 27746aaf1b20172f0859546c4a3e82eca459f680 (diff) | |
selftests/bpf: Improve reliability of test_perf_branches_no_hw()
Currently, test_perf_branches_no_hw() relies on the busy loop within
test_perf_branches_common() being slow enough to allow at least one
perf event sample tick to occur before starting to tear down the
backing perf event BPF program. With a relatively small fixed
iteration count of 1,000,000, this is not guaranteed on modern fast
CPUs, resulting in the test run to subsequently fail with the
following:
bpf_testmod.ko is already unloaded.
Loading bpf_testmod.ko...
Successfully loaded bpf_testmod.ko.
test_perf_branches_common:PASS:test_perf_branches_load 0 nsec
test_perf_branches_common:PASS:attach_perf_event 0 nsec
test_perf_branches_common:PASS:set_affinity 0 nsec
check_good_sample:PASS:output not valid 0 nsec
check_good_sample:PASS:read_branches_size 0 nsec
check_good_sample:PASS:read_branches_stack 0 nsec
check_good_sample:PASS:read_branches_stack 0 nsec
check_good_sample:PASS:read_branches_global 0 nsec
check_good_sample:PASS:read_branches_global 0 nsec
check_good_sample:PASS:read_branches_size 0 nsec
test_perf_branches_no_hw:PASS:perf_event_open 0 nsec
test_perf_branches_common:PASS:test_perf_branches_load 0 nsec
test_perf_branches_common:PASS:attach_perf_event 0 nsec
test_perf_branches_common:PASS:set_affinity 0 nsec
check_bad_sample:FAIL:output not valid no valid sample from prog
Summary: 0/1 PASSED, 0 SKIPPED, 1 FAILED
Successfully unloaded bpf_testmod.ko.
On a modern CPU (i.e. one with a 3.5 GHz clock rate), executing 1
million increments of a volatile integer can take significantly less
than 1 millisecond. If the spin loop and detachment of the perf event
BPF program elapses before the first 1 ms sampling interval elapses,
the perf event will never end up firing. Fix this by bumping the loop
iteration counter a little within test_perf_branches_common(), along
with ensuring adding another loop termination condition which is
directly influenced by the backing perf event BPF program
executing. Notably, a concious decision was made to not adjust the
sample_freq value as that is just not a reliable way to go about
fixing the problem. It effectively still leaves the race window open.
Fixes: 67306f84ca78c ("selftests/bpf: Add bpf_read_branch_records() selftest")
Signed-off-by: Matt Bobrowski <mattbobrowski@google.com>
Reviewed-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/r/20251119143540.2911424-1-mattbobrowski@google.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Diffstat (limited to 'tools/testing/selftests/bpf/progs/test_perf_branches.c')
| -rw-r--r-- | tools/testing/selftests/bpf/progs/test_perf_branches.c | 3 |
1 files changed, 3 insertions, 0 deletions
diff --git a/tools/testing/selftests/bpf/progs/test_perf_branches.c b/tools/testing/selftests/bpf/progs/test_perf_branches.c index a1ccc831c882..05ac9410cd68 100644 --- a/tools/testing/selftests/bpf/progs/test_perf_branches.c +++ b/tools/testing/selftests/bpf/progs/test_perf_branches.c @@ -8,6 +8,7 @@ #include <bpf/bpf_tracing.h> int valid = 0; +int run_cnt = 0; int required_size_out = 0; int written_stack_out = 0; int written_global_out = 0; @@ -24,6 +25,8 @@ int perf_branches(void *ctx) __u64 entries[4 * 3] = {0}; int required_size, written_stack, written_global; + ++run_cnt; + /* write to stack */ written_stack = bpf_read_branch_records(ctx, entries, sizeof(entries), 0); /* ignore spurious events */ |