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selftests/bpf: Fix flaky send_signal test

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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.yml

Signed-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
This commit is contained in:
Yonghong Song 2021-08-17 12:09:23 -07:00 committed by Andrii Nakryiko
parent 6f6cc42645
commit b16ac5bf73
1 changed files with 16 additions and 0 deletions
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16
tools/testing/selftests/bpf/prog_tests/send_signal.c
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@ -1,5 +1,7 @@
// 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);