samples/bpf: add bpf map stress test

this test calls bpf programs from different contexts:
from inside of slub, from rcu, from pretty much everywhere,
since it kprobes all spin_lock functions.
It stresses the bpf hash and percpu map pre-allocation,
deallocation logic and call_rcu mechanisms.
User space part adding more stress by walking and deleting map elements.

Note that due to nature bpf_load.c the earlier kprobe+bpf programs are
already active while loader loads new programs, creates new kprobes and
attaches them.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

samples/bpf/spintest_user.c

@@ -0,0 +1,50 @@
+#include <stdio.h>
+#include <unistd.h>
+#include <linux/bpf.h>
+#include <string.h>
+#include <assert.h>
+#include <sys/resource.h>
+#include "libbpf.h"
+#include "bpf_load.h"
+
+int main(int ac, char **argv)
+{
+	struct rlimit r = {RLIM_INFINITY, RLIM_INFINITY};
+	long key, next_key, value;
+	char filename[256];
+	struct ksym *sym;
+	int i;
+
+	snprintf(filename, sizeof(filename), "%s_kern.o", argv[0]);
+	setrlimit(RLIMIT_MEMLOCK, &r);
+
+	if (load_kallsyms()) {
+		printf("failed to process /proc/kallsyms\n");
+		return 2;
+	}
+
+	if (load_bpf_file(filename)) {
+		printf("%s", bpf_log_buf);
+		return 1;
+	}
+
+	for (i = 0; i < 5; i++) {
+		key = 0;
+		printf("kprobing funcs:");
+		while (bpf_get_next_key(map_fd[0], &key, &next_key) == 0) {
+			bpf_lookup_elem(map_fd[0], &next_key, &value);
+			assert(next_key == value);
+			sym = ksym_search(value);
+			printf(" %s", sym->name);
+			key = next_key;
+		}
+		if (key)
+			printf("\n");
+		key = 0;
+		while (bpf_get_next_key(map_fd[0], &key, &next_key) == 0)
+			bpf_delete_elem(map_fd[0], &next_key);
+		sleep(1);
+	}
+
+	return 0;
+}