Re: [PATCH] perf test: Handle fd gaps in test__dso_data_reopen

[Michael Petlan <mpetlan@redhat.com>]

On Fri, 25 Jun 2021, Eirik Fuller wrote:
> https://github.com/beaker-project/restraint/issues/215 describes a file
> descriptor leak which revealed the test failure described here.
> 
> The 'DSO data reopen' perf test assumes that RLIMIT_NOFILE limits the
> number of open file descriptors, but it actually limits newly opened
> file descriptors. When the file descriptor limit is reduced, file
> descriptors already open remain open regardless of the new limit. This
> test failure does not occur if open file descriptors are contiguous,
> beginning at zero.
> 
> The following command triggers this perf test failure.
> 
> perf test 'DSO data reopen' 3>/dev/null 8>/dev/null
> 
> This patch determines the file descriptor limit by opening four files
> and then closing them. The limit is set to the fourth file descriptor,
> leaving only the first three available because any newly opened file
> descriptor must be less than the limit.
> 
> Signed-off-by: Eirik Fuller <efuller@redhat.com>

Acked-by: Michael Petlan <mpetlan@redhat.com>

> ---
>  tools/perf/tests/dso-data.c | 14 ++++++++++++--
>  1 file changed, 12 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/perf/tests/dso-data.c b/tools/perf/tests/dso-data.c
> index 627c1aaf1c9e..43e1b01e5afc 100644
> --- a/tools/perf/tests/dso-data.c
> +++ b/tools/perf/tests/dso-data.c
> @@ -308,10 +308,20 @@ int test__dso_data_cache(struct test *test __maybe_unused, int subtest __maybe_u
>  	return 0;
>  }
>  
> +static long new_limit(int count)
> +{
> +	int fd = open("/dev/null", O_RDONLY);
> +	long ret = fd;
> +	if (count > 0)
> +		ret = new_limit(--count);
> +	close(fd);
> +	return ret;
> +}
> +
>  int test__dso_data_reopen(struct test *test __maybe_unused, int subtest __maybe_unused)
>  {
>  	struct machine machine;
> -	long nr_end, nr = open_files_cnt();
> +	long nr_end, nr = open_files_cnt(), lim = new_limit(3);
>  	int fd, fd_extra;
>  
>  #define dso_0 (dsos[0])
> @@ -334,7 +344,7 @@ int test__dso_data_reopen(struct test *test __maybe_unused, int subtest __maybe_
>  
>  	/* Make sure we are able to open 3 fds anyway */
>  	TEST_ASSERT_VAL("failed to set file limit",
> -			!set_fd_limit((nr + 3)));
> +			!set_fd_limit((lim)));
>  
>  	TEST_ASSERT_VAL("failed to create dsos\n", !dsos__create(3, TEST_FILE_SIZE));
>  
> -- 
> 2.27.0
> 
>