6.18-stable review patch. If anyone has any objections, please let me know.
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From: Matt Bobrowski mattbobrowski@google.com
[ Upstream commit ae24fc8a16b0481ea8c5acbc66453c49ec0431c4 ]
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 Signed-off-by: Sasha Levin sashal@kernel.org --- .../selftests/bpf/prog_tests/perf_branches.c | 16 ++++++++++++++-- .../selftests/bpf/progs/test_perf_branches.c | 3 +++ 2 files changed, 17 insertions(+), 2 deletions(-)
diff --git a/tools/testing/selftests/bpf/prog_tests/perf_branches.c b/tools/testing/selftests/bpf/prog_tests/perf_branches.c index 06c7986131d96..0a7ef770c487c 100644 --- a/tools/testing/selftests/bpf/prog_tests/perf_branches.c +++ b/tools/testing/selftests/bpf/prog_tests/perf_branches.c @@ -15,6 +15,10 @@ static void check_good_sample(struct test_perf_branches *skel) int pbe_size = sizeof(struct perf_branch_entry); int duration = 0;
+ if (CHECK(!skel->bss->run_cnt, "invalid run_cnt", + "checked sample validity before prog run")) + return; + if (CHECK(!skel->bss->valid, "output not valid", "no valid sample from prog")) return; @@ -45,6 +49,10 @@ static void check_bad_sample(struct test_perf_branches *skel) int written_stack = skel->bss->written_stack_out; int duration = 0;
+ if (CHECK(!skel->bss->run_cnt, "invalid run_cnt", + "checked sample validity before prog run")) + return; + if (CHECK(!skel->bss->valid, "output not valid", "no valid sample from prog")) return; @@ -83,8 +91,12 @@ static void test_perf_branches_common(int perf_fd, err = pthread_setaffinity_np(pthread_self(), sizeof(cpu_set), &cpu_set); if (CHECK(err, "set_affinity", "cpu #0, err %d\n", err)) goto out_destroy; - /* spin the loop for a while (random high number) */ - for (i = 0; i < 1000000; ++i) + + /* Spin the loop for a while by using a high iteration count, and by + * checking whether the specific run count marker has been explicitly + * incremented at least once by the backing perf_event BPF program. + */ + for (i = 0; i < 100000000 && !*(volatile int *)&skel->bss->run_cnt; ++i) ++j;
test_perf_branches__detach(skel); diff --git a/tools/testing/selftests/bpf/progs/test_perf_branches.c b/tools/testing/selftests/bpf/progs/test_perf_branches.c index a1ccc831c882f..05ac9410cd68c 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 */