2 commits

Author	SHA1	Message	Date
Viktor Malik	de07b18289	selftests/bpf: Fix string read in strncmp benchmark ... The strncmp benchmark uses the bpf_strncmp helper and a hand-written loop to compare two strings. The values of the strings are filled from userspace. One of the strings is non-const (in .bss) while the other is const (in .rodata) since that is the requirement of bpf_strncmp. The problem is that in the hand-written loop, Clang optimizes the reads from the const string to always return 0 which breaks the benchmark. Use barrier_var to prevent the optimization. The effect can be seen on the strncmp-no-helper variant. Before this change: # ./bench strncmp-no-helper Setting up benchmark 'strncmp-no-helper'... Benchmark 'strncmp-no-helper' started. Iter 0 (112.309us): hits 0.000M/s ( 0.000M/prod), drops 0.000M/s, total operations 0.000M/s Iter 1 (-23.238us): hits 0.000M/s ( 0.000M/prod), drops 0.000M/s, total operations 0.000M/s Iter 2 ( 58.994us): hits 0.000M/s ( 0.000M/prod), drops 0.000M/s, total operations 0.000M/s Iter 3 (-30.466us): hits 0.000M/s ( 0.000M/prod), drops 0.000M/s, total operations 0.000M/s Iter 4 ( 29.996us): hits 0.000M/s ( 0.000M/prod), drops 0.000M/s, total operations 0.000M/s Iter 5 ( 16.949us): hits 0.000M/s ( 0.000M/prod), drops 0.000M/s, total operations 0.000M/s Iter 6 (-60.035us): hits 0.000M/s ( 0.000M/prod), drops 0.000M/s, total operations 0.000M/s Summary: hits 0.000 ± 0.000M/s ( 0.000M/prod), drops 0.000 ± 0.000M/s, total operations 0.000 ± 0.000M/s After this change: # ./bench strncmp-no-helper Setting up benchmark 'strncmp-no-helper'... Benchmark 'strncmp-no-helper' started. Iter 0 ( 77.711us): hits 5.534M/s ( 5.534M/prod), drops 0.000M/s, total operations 5.534M/s Iter 1 ( 11.215us): hits 6.006M/s ( 6.006M/prod), drops 0.000M/s, total operations 6.006M/s Iter 2 (-14.253us): hits 5.931M/s ( 5.931M/prod), drops 0.000M/s, total operations 5.931M/s Iter 3 ( 59.087us): hits 6.005M/s ( 6.005M/prod), drops 0.000M/s, total operations 6.005M/s Iter 4 (-21.379us): hits 6.010M/s ( 6.010M/prod), drops 0.000M/s, total operations 6.010M/s Iter 5 (-20.310us): hits 5.861M/s ( 5.861M/prod), drops 0.000M/s, total operations 5.861M/s Iter 6 ( 53.937us): hits 6.004M/s ( 6.004M/prod), drops 0.000M/s, total operations 6.004M/s Summary: hits 5.969 ± 0.061M/s ( 5.969M/prod), drops 0.000 ± 0.000M/s, total operations 5.969 ± 0.061M/s Fixes: 9c42652f8b ("selftests/bpf: Add benchmark for bpf_strncmp() helper") Suggested-by: Andrii Nakryiko <andrii@kernel.org> Signed-off-by: Viktor Malik <vmalik@redhat.com> Signed-off-by: Andrii Nakryiko <andrii@kernel.org> Acked-by: Hou Tao <houtao1@huawei.com> Link: https://lore.kernel.org/bpf/20250313122852.1365202-1-vmalik@redhat.com Signed-off-by: Alexei Starovoitov <ast@kernel.org>	2025-03-15 11:49:24 -07:00
Hou Tao	9c42652f8b	selftests/bpf: Add benchmark for bpf_strncmp() helper ... Add benchmark to compare the performance between home-made strncmp() in bpf program and bpf_strncmp() helper. In summary, the performance win of bpf_strncmp() under x86-64 is greater than 18% when the compared string length is greater than 64, and is 179% when the length is 4095. Under arm64 the performance win is even bigger: 33% when the length is greater than 64 and 600% when the length is 4095. The following is the details: no-helper-X: use home-made strncmp() to compare X-sized string helper-Y: use bpf_strncmp() to compare Y-sized string Under x86-64: no-helper-1 3.504 ± 0.000M/s (drops 0.000 ± 0.000M/s) helper-1 3.347 ± 0.001M/s (drops 0.000 ± 0.000M/s) no-helper-8 3.357 ± 0.001M/s (drops 0.000 ± 0.000M/s) helper-8 3.307 ± 0.001M/s (drops 0.000 ± 0.000M/s) no-helper-32 3.064 ± 0.000M/s (drops 0.000 ± 0.000M/s) helper-32 3.253 ± 0.001M/s (drops 0.000 ± 0.000M/s) no-helper-64 2.563 ± 0.001M/s (drops 0.000 ± 0.000M/s) helper-64 3.040 ± 0.001M/s (drops 0.000 ± 0.000M/s) no-helper-128 1.975 ± 0.000M/s (drops 0.000 ± 0.000M/s) helper-128 2.641 ± 0.000M/s (drops 0.000 ± 0.000M/s) no-helper-512 0.759 ± 0.000M/s (drops 0.000 ± 0.000M/s) helper-512 1.574 ± 0.000M/s (drops 0.000 ± 0.000M/s) no-helper-2048 0.329 ± 0.000M/s (drops 0.000 ± 0.000M/s) helper-2048 0.602 ± 0.000M/s (drops 0.000 ± 0.000M/s) no-helper-4095 0.117 ± 0.000M/s (drops 0.000 ± 0.000M/s) helper-4095 0.327 ± 0.000M/s (drops 0.000 ± 0.000M/s) Under arm64: no-helper-1 2.806 ± 0.004M/s (drops 0.000 ± 0.000M/s) helper-1 2.819 ± 0.002M/s (drops 0.000 ± 0.000M/s) no-helper-8 2.797 ± 0.109M/s (drops 0.000 ± 0.000M/s) helper-8 2.786 ± 0.025M/s (drops 0.000 ± 0.000M/s) no-helper-32 2.399 ± 0.011M/s (drops 0.000 ± 0.000M/s) helper-32 2.703 ± 0.002M/s (drops 0.000 ± 0.000M/s) no-helper-64 2.020 ± 0.015M/s (drops 0.000 ± 0.000M/s) helper-64 2.702 ± 0.073M/s (drops 0.000 ± 0.000M/s) no-helper-128 1.604 ± 0.001M/s (drops 0.000 ± 0.000M/s) helper-128 2.516 ± 0.002M/s (drops 0.000 ± 0.000M/s) no-helper-512 0.699 ± 0.000M/s (drops 0.000 ± 0.000M/s) helper-512 2.106 ± 0.003M/s (drops 0.000 ± 0.000M/s) no-helper-2048 0.215 ± 0.000M/s (drops 0.000 ± 0.000M/s) helper-2048 1.223 ± 0.003M/s (drops 0.000 ± 0.000M/s) no-helper-4095 0.112 ± 0.000M/s (drops 0.000 ± 0.000M/s) helper-4095 0.796 ± 0.000M/s (drops 0.000 ± 0.000M/s) Signed-off-by: Hou Tao <houtao1@huawei.com> Signed-off-by: Alexei Starovoitov <ast@kernel.org> Link: https://lore.kernel.org/bpf/20211210141652.877186-4-houtao1@huawei.com	2021-12-11 17:40:23 -08:00