On 5/8/20 4:20 PM, Andrii Nakryiko wrote:
It is sometimes desirable to be able to trigger BPF program from user-space with minimal overhead. sys_enter would seem to be a good candidate, yet in
Probably "with minimal external noise"? Typically, overhead means the overhead from test infrastructure itself?
a lot of cases there will be a lot of noise from syscalls triggered by other processes on the system. So while searching for low-overhead alternative, I've stumbled upon getpgid() syscall, which seems to be specific enough to not suffer from accidental syscall by other apps. This set of benchmarks compares tp, raw_tp w/ filtering by syscall ID, kprobe, fentry and fmod_ret with returning error (so that syscall would not be executed), to determine the lowest-overhead way. Here are results on my machine (using benchs/run_bench_trigger.sh script): base : 9.200 ± 0.319M/s tp : 6.690 ± 0.125M/s rawtp : 8.571 ± 0.214M/s kprobe : 6.431 ± 0.048M/s fentry : 8.955 ± 0.241M/s fmodret : 8.903 ± 0.135M/s
The relative ranking of different approaches is still similar to patch #2. But this patch reinforces that benchmarking really needs to reduce the noise to get highest number.
So it seems like fmodret doesn't give much benefit for such lightweight syscall. Raw tracepoint is pretty decent despite additional filtering logic, but it will be called for any other syscall in the system, which rules it out. Fentry, though, seems to be adding the least amoung of overhead and achieves 97.3% of performance of baseline no-BPF-attached syscall. Using getpgid() seems to be preferable to set_task_comm() approach from test_overhead, as it's about 2.35x faster in a baseline performance. Acked-by: John Fastabend <john.fastabend@xxxxxxxxx> Signed-off-by: Andrii Nakryiko <andriin@xxxxxx>
Acked-by: Yonghong Song <yhs@xxxxxx>
--- tools/testing/selftests/bpf/Makefile | 4 +- tools/testing/selftests/bpf/bench.c | 12 ++ .../selftests/bpf/benchs/bench_trigger.c | 167 ++++++++++++++++++ .../selftests/bpf/benchs/run_bench_trigger.sh | 9 + .../selftests/bpf/progs/trigger_bench.c | 47 +++++ 5 files changed, 238 insertions(+), 1 deletion(-) create mode 100644 tools/testing/selftests/bpf/benchs/bench_trigger.c create mode 100755 tools/testing/selftests/bpf/benchs/run_bench_trigger.sh create mode 100644 tools/testing/selftests/bpf/progs/trigger_bench.c
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