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    Subject[PATCH 5.13 283/380] selftests/bpf: Fix flaky send_signal test
    Date
    From: Yonghong Song <yhs@fb.com>

    [ Upstream commit b16ac5bf732a5e23d164cf908ec7742d6a6120d3 ]

    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
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    .../selftests/bpf/prog_tests/send_signal.c | 16 ++++++++++++++++
    1 file changed, 16 insertions(+)

    diff --git a/tools/testing/selftests/bpf/prog_tests/send_signal.c b/tools/testing/selftests/bpf/prog_tests/send_signal.c
    index 7043e6ded0e6..75b72c751772 100644
    --- a/tools/testing/selftests/bpf/prog_tests/send_signal.c
    +++ b/tools/testing/selftests/bpf/prog_tests/send_signal.c
    @@ -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"

    static volatile int sigusr1_received = 0;
    @@ -41,12 +43,23 @@ 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 */
    CHECK(write(pipe_c2p[1], buf, 1) != 1, "pipe_write", "err %d\n", -errno);

    @@ -62,6 +75,9 @@ static void test_send_signal_common(struct perf_event_attr *attr,
    /* wait for parent notification and exit */
    CHECK(read(pipe_p2c[0], buf, 1) != 1, "pipe_read", "err %d\n", -errno);

    + /* restore the old priority */
    + ASSERT_OK(setpriority(PRIO_PROCESS, 0, old_prio), "setpriority");
    +
    close(pipe_c2p[1]);
    close(pipe_p2c[0]);
    exit(0);
    --
    2.30.2


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