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 <vmalik@xxxxxxxxxx>
---
tools/testing/selftests/bpf/progs/strncmp_bench.c | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)
diff --git a/tools/testing/selftests/bpf/progs/strncmp_bench.c b/tools/testing/selftests/bpf/progs/strncmp_bench.c
index 18373a7df76e..92a828a1ebea 100644
--- a/tools/testing/selftests/bpf/progs/strncmp_bench.c
+++ b/tools/testing/selftests/bpf/progs/strncmp_bench.c
@@ -9,7 +9,7 @@
/* Will be updated by benchmark before program loading */
const volatile unsigned int cmp_str_len = 1;
-const char target[STRNCMP_STR_SZ];
+const volatile char target[STRNCMP_STR_SZ];
long hits = 0;
char str[STRNCMP_STR_SZ];
@@ -17,7 +17,7 @@ char str[STRNCMP_STR_SZ];
char _license[] SEC("license") = "GPL";
static __always_inline int local_strncmp(const char *s1, unsigned int sz,
- const char *s2)
+ const volatile char *s2)
{
int ret = 0;
unsigned int i;
@@ -43,7 +43,7 @@ int strncmp_no_helper(void *ctx)
SEC("tp/syscalls/sys_enter_getpgid")
int strncmp_helper(void *ctx)
{
- if (bpf_strncmp(str, cmp_str_len + 1, target) < 0)
+ if (bpf_strncmp(str, cmp_str_len + 1, (const char *)target) < 0)
__sync_add_and_fetch(&hits, 1);
return 0;
}
--
2.48.1
