Re: [RFC PATCH bpf-next 2/2] selftests/bpf: add benchmark bpf_strcmp

On Sat, Nov 06, 2021 at 09:28:22PM +0800, Hou Tao wrote:

The benchmark runs a loop 5000 times. In the loop it reads the file name
from kprobe argument into stack by using bpf_probe_read_kernel_str(),
and compares the file name with a target character or string.

Three cases are compared: only compare one character, compare the whole
string by a home-made strncmp() and compare the whole string by
bpf_strcmp().

The following is the result:

x86-64 host:

one character: 2613499 ns
whole str by strncmp: 2920348 ns
whole str by helper: 2779332 ns

arm64 host:

one character: 3898867 ns
whole str by strncmp: 4396787 ns
whole str by helper: 3968113 ns

Compared with home-made strncmp, the performance of bpf_strncmp helper
improves 80% under x86-64 and 600% under arm64. The big performance win
on arm64 may comes from its arch-optimized strncmp().

80% and 600% improvement?!
I don't understand how this math works.

Why one char is barely different in total nsec than the whole string?
The string shouldn't miscompare on the first char as far as I understand the test.