selftests/bpf: Fix flaky send_signal test

libbpf CI has reported send_signal test is flaky although
I am not able to reproduce it in my local environment.
But I am able to reproduce with on-demand libbpf CI ([1]).

Through code analysis, the following is possible reason.
The failed subtest runs bpf program in softirq environment.
Since bpf_send_signal() only sends to a fork of "test_progs"
process. If the underlying current task is
not "test_progs", bpf_send_signal() will not be triggered
and the subtest will fail.

To reduce the chances where the underlying process is not
the intended one, this patch boosted scheduling priority to
-20 (highest allowed by setpriority() call). And I did
10 runs with on-demand libbpf CI with this patch and I
didn't observe any failures.

 [1] https://github.com/libbpf/libbpf/actions/workflows/ondemand.ymlSigned-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210817190923.3186725-1-yhs@fb.com

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

 // SPDX-License-Identifier: GPL-2.0
 #include <test_progs.h>
+#include <sys/time.h>
+#include <sys/resource.h>
 #include "test_send_signal_kern.skel.h"
 
 int sigusr1_received = 0;
@@ -37,12 +39,23 @@ static void test_send_signal_common(struct perf_event_attr *attr,
 	}
 
 	if (pid == 0) {
+		int old_prio;
+
 		/* install signal handler and notify parent */
 		signal(SIGUSR1, sigusr1_handler);
 
 		close(pipe_c2p[0]); /* close read */
 		close(pipe_p2c[1]); /* close write */
 
+		/* boost with a high priority so we got a higher chance
+		 * that if an interrupt happens, the underlying task
+		 * is this process.
+		 */
+		errno = 0;
+		old_prio = getpriority(PRIO_PROCESS, 0);
+		ASSERT_OK(errno, "getpriority");
+		ASSERT_OK(setpriority(PRIO_PROCESS, 0, -20), "setpriority");
+
 		/* notify parent signal handler is installed */
 		ASSERT_EQ(write(pipe_c2p[1], buf, 1), 1, "pipe_write");
 
@@ -58,6 +71,9 @@ static void test_send_signal_common(struct perf_event_attr *attr,
 		/* wait for parent notification and exit */
 		ASSERT_EQ(read(pipe_p2c[0], buf, 1), 1, "pipe_read");
 
+		/* restore the old priority */
+		ASSERT_OK(setpriority(PRIO_PROCESS, 0, old_prio), "setpriority");
+
 		close(pipe_c2p[1]);
 		close(pipe_p2c[0]);
 		exit(0);