selftests/bpf: Add send_signal_sched_switch test

Added one test, send_signal_sched_switch, to test bpf_send_signal()
helper triggered by sched/sched_switch tracepoint. This test can be used
to verify kernel deadlocks fixed by the previous commit. The test itself
is heavily borrowed from Commit eac9153f ("bpf/stackmap: Fix deadlock
with rq_lock in bpf_get_stack()").
Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Cc: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20200304191105.2796601-1-yhs@fb.com

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

+// SPDX-License-Identifier: GPL-2.0
+#include <test_progs.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/mman.h>
+#include <pthread.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+#include "test_send_signal_kern.skel.h"
+
+static void sigusr1_handler(int signum)
+{
+}
+
+#define THREAD_COUNT 100
+
+static void *worker(void *p)
+{
+	int i;
+
+	for ( i = 0; i < 1000; i++)
+		usleep(1);
+
+	return NULL;
+}
+
+void test_send_signal_sched_switch(void)
+{
+	struct test_send_signal_kern *skel;
+	pthread_t threads[THREAD_COUNT];
+	u32 duration = 0;
+	int i, err;
+
+	signal(SIGUSR1, sigusr1_handler);
+
+	skel = test_send_signal_kern__open_and_load();
+	if (CHECK(!skel, "skel_open_and_load", "skeleton open_and_load failed\n"))
+		return;
+
+	skel->bss->pid = getpid();
+	skel->bss->sig = SIGUSR1;
+
+	err = test_send_signal_kern__attach(skel);
+	if (CHECK(err, "skel_attach", "skeleton attach failed\n"))
+		goto destroy_skel;
+
+	for (i = 0; i < THREAD_COUNT; i++) {
+		err = pthread_create(threads + i, NULL, worker, NULL);
+		if (CHECK(err, "pthread_create", "Error creating thread, %s\n",
+			  strerror(errno)))
+			goto destroy_skel;
+	}
+
+	for (i = 0; i < THREAD_COUNT; i++)
+		pthread_join(threads[i], NULL);
+
+destroy_skel:
+	test_send_signal_kern__destroy(skel);
+}

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

@@ -31,6 +31,12 @@ int send_signal_tp(void *ctx)
 	return bpf_send_signal_test(ctx);
 }
 
+SEC("tracepoint/sched/sched_switch")
+int send_signal_tp_sched(void *ctx)
+{
+	return bpf_send_signal_test(ctx);
+}
+
 SEC("perf_event")
 int send_signal_perf(void *ctx)
 {