selftests/bpf: add usage example for cpu time counter kfuncs

The selftest provides an example of how to measure the latency of bpf
kfunc/helper call using time stamp counter and how to convert measured
value into nanoseconds.

Signed-off-by: Vadim Fedorenko <[email protected]>

Author: vvfedorenko

tools/testing/selftests/bpf/prog_tests/test_cpu_cycles.c

@@ -0,0 +1,35 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2024 Meta Inc. */
+
+#include <test_progs.h>
+#include "test_cpu_cycles.skel.h"
+
+static void cpu_cycles(void)
+{
+	LIBBPF_OPTS(bpf_test_run_opts, opts);
+	struct test_cpu_cycles *skel;
+	int err, pfd;
+
+	skel = test_cpu_cycles__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "test_cpu_cycles open and load"))
+		return;
+
+	pfd = bpf_program__fd(skel->progs.bpf_cpu_cycles);
+	if (!ASSERT_GT(pfd, 0, "test_cpu_cycles fd"))
+		goto fail;
+
+	err = bpf_prog_test_run_opts(pfd, &opts);
+	if (!ASSERT_OK(err, "test_cpu_cycles test run"))
+		goto fail;
+
+	ASSERT_NEQ(skel->bss->cycles, 0, "test_cpu_cycles 0 cycles");
+	ASSERT_NEQ(skel->bss->ns, 0, "test_cpu_cycles 0 ns");
+fail:
+	test_cpu_cycles__destroy(skel);
+}
+
+void test_cpu_cycles(void)
+{
+	if (test__start_subtest("cpu_cycles"))
+		cpu_cycles();
+}

tools/testing/selftests/bpf/progs/test_cpu_cycles.c

@@ -0,0 +1,25 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2024 Meta Inc. */
+
+#include "vmlinux.h"
+#include <bpf/bpf_helpers.h>
+
+extern u64 bpf_cpu_time_counter_to_ns(u64 cycles) __weak __ksym;
+extern u64 bpf_get_cpu_time_counter(void) __weak __ksym;
+
+__u64 cycles, ns;
+
+SEC("syscall")
+int bpf_cpu_cycles(void)
+{
+	struct bpf_pidns_info pidns;
+	__u64 start;
+
+	start = bpf_get_cpu_time_counter();
+	bpf_get_ns_current_pid_tgid(0, 0, &pidns, sizeof(struct bpf_pidns_info));
+	cycles = bpf_get_cpu_time_counter() - start;
+	ns = bpf_cpu_time_counter_to_ns(cycles);
+	return 0;
+}
+
+char _license[] SEC("license") = "GPL";