[PATCH] selftests/seccomp: Pin benchmark to single CPU

[Kees Cook]

The seccomp benchmark test (for validating the benefit of bitmaps) can
be sensitive to scheduling speed, so pin the process to a single CPU,
which appears to significantly improve reliability.

Reported-by: kernel test robot <oliver.sang@xxxxxxxxx>
Closes: https://lore.kernel.org/oe-lkp/202402061002.3a8722fd-oliver.sang@xxxxxxxxx
Cc: Mark Brown <broonie@xxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Cc: Will Drewry <wad@xxxxxxxxxxxx>
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 .../selftests/seccomp/seccomp_benchmark.c     | 27 +++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/tools/testing/selftests/seccomp/seccomp_benchmark.c b/tools/testing/selftests/seccomp/seccomp_benchmark.c
index 5b5c9d558dee..d0b733e708cc 100644
--- a/tools/testing/selftests/seccomp/seccomp_benchmark.c
+++ b/tools/testing/selftests/seccomp/seccomp_benchmark.c
@@ -4,7 +4,9 @@
  */
 #define _GNU_SOURCE
 #include <assert.h>
+#include <err.h>
 #include <limits.h>
+#include <sched.h>
 #include <stdbool.h>
 #include <stddef.h>
 #include <stdio.h>
@@ -119,6 +121,29 @@ long compare(const char *name_one, const char *name_eval, const char *name_two,
 	return good ? 0 : 1;
 }
 
+/* Pin to a single CPU so the benchmark won't bounce around the system. */
+void affinity(void)
+{
+	long cpu;
+	ulong ncores = sysconf(_SC_NPROCESSORS_CONF);
+	cpu_set_t *setp = CPU_ALLOC(ncores);
+	ulong setsz = CPU_ALLOC_SIZE(ncores);
+
+	/* Set from highest CPU down. */
+	for (cpu = ncores - 1; cpu >= 0; cpu--) {
+		CPU_ZERO_S(setsz, setp);
+		CPU_SET_S(cpu, setsz, setp);
+		if (sched_setaffinity(getpid(), setsz, setp) == -1)
+			continue;
+		printf("Pinned to CPU %lu of %lu\n", cpu + 1, ncores);
+		goto out;
+	}
+	fprintf(stderr, "Could not set CPU affinity -- calibration may not work well");
+
+out:
+	CPU_FREE(setp);
+}
+
 int main(int argc, char *argv[])
 {
 	struct sock_filter bitmap_filter[] = {
@@ -153,6 +178,8 @@ int main(int argc, char *argv[])
 	system("grep -H . /proc/sys/net/core/bpf_jit_enable");
 	system("grep -H . /proc/sys/net/core/bpf_jit_harden");
 
+	affinity();
+
 	if (argc > 1)
 		samples = strtoull(argv[1], NULL, 0);
 	else
-- 
2.34.1