selftests/bpf: Fix string read in strncmp benchmark #8663
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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.
Mark the const string as volatile to avoid that.
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 (8440.107us): hits 0.000M/s ( 0.000M/prod), drops 0.000M/s, total operations 0.000M/s
Iter 1 (73909.374us): hits 0.000M/s ( 0.000M/prod), drops 0.000M/s, total operations 0.000M/s
Iter 2 (-8140.994us): hits 0.000M/s ( 0.000M/prod), drops 0.000M/s, total operations 0.000M/s
Iter 3 (3094.474us): hits 0.000M/s ( 0.000M/prod), drops 0.000M/s, total operations 0.000M/s
Iter 4 (-2828.468us): hits 0.000M/s ( 0.000M/prod), drops 0.000M/s, total operations 0.000M/s
Iter 5 (2635.595us): hits 0.000M/s ( 0.000M/prod), drops 0.000M/s, total operations 0.000M/s
Iter 6 (-306.478us): 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 (21180.011us): hits 5.320M/s ( 5.320M/prod), drops 0.000M/s, total operations 5.320M/s
Iter 1 (-692.499us): hits 5.246M/s ( 5.246M/prod), drops 0.000M/s, total operations 5.246M/s
Iter 2 (-704.751us): hits 5.332M/s ( 5.332M/prod), drops 0.000M/s, total operations 5.332M/s
Iter 3 (62057.929us): hits 5.299M/s ( 5.299M/prod), drops 0.000M/s, total operations 5.299M/s
Iter 4 (-7981.421us): hits 5.303M/s ( 5.303M/prod), drops 0.000M/s, total operations 5.303M/s
Iter 5 (3500.341us): hits 5.306M/s ( 5.306M/prod), drops 0.000M/s, total operations 5.306M/s
Iter 6 (-3851.046us): hits 5.264M/s ( 5.264M/prod), drops 0.000M/s, total operations 5.264M/s
Summary: hits 5.338 ± 0.147M/s ( 5.338M/prod), drops 0.000 ± 0.000M/s, total operations 5.338 ± 0.147M/s
Signed-off-by: Viktor Malik <[email protected]>
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At least one diff in series https://patchwork.kernel.org/project/netdevbpf/list/?series=943010 expired. Closing PR. |
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Pull request for series with
subject: selftests/bpf: Fix string read in strncmp benchmark
version: 1
url: https://patchwork.kernel.org/project/netdevbpf/list/?series=943010