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From:   Ryan Roberts <ryan.roberts@arm.com>
To:     "Andrew Morton" <akpm@linux-foundation.org>,
        "Shuah Khan" <shuah@kernel.org>,
        =?UTF-8?q?J=C3=A9r=C3=B4me=20Glisse?= <jglisse@redhat.com>,
        "David Hildenbrand" <david@redhat.com>,
        "Mark Brown" <broonie@kernel.org>,
        "John Hubbard" <jhubbard@nvidia.com>,
        "Florent Revest" <revest@chromium.org>,
        Peter Xu <peterx@redhat.com>
Cc:     Ryan Roberts <ryan.roberts@arm.com>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, linux-kselftest@vger.kernel.org
Subject: [PATCH v2 1/8] selftests: Line buffer test program's stdout
Date:   Mon, 17 Jul 2023 11:31:45 +0100
Message-Id: <20230717103152.202078-2-ryan.roberts@arm.com>
In-Reply-To: <20230717103152.202078-1-ryan.roberts@arm.com>
References: <20230717103152.202078-1-ryan.roberts@arm.com>
MIME-Version: 1.0
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The selftests runner pipes the test program's stdout to tap_prefix. The
presence of the pipe means that the test program sets its stdout to be
fully buffered (as aposed to line buffered when directly connected to
the terminal). The block buffering means that there is often content in
the buffer at fork() time, which causes the output to end up duplicated.
This was causing problems for mm:cow where test results were duplicated
20-30x.

Solve this by using `stdbuf`, when available to force the test program
to use line buffered mode. This means previously printf'ed results are
flushed out of the program before any fork().

Additionally, explicitly set line buffer mode in ksft_print_header(),
which means that all test programs that use the ksft framework will
benefit even if stdbuf is not present on the system.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
---
 tools/testing/selftests/kselftest.h         | 9 +++++++++
 tools/testing/selftests/kselftest/runner.sh | 7 +++++--
 2 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/kselftest.h b/tools/testing/selftests/kselftest.h
index 829be379545a..529d29a35900 100644
--- a/tools/testing/selftests/kselftest.h
+++ b/tools/testing/selftests/kselftest.h
@@ -113,6 +113,15 @@ static inline int ksft_get_error_cnt(void) { return ksft_cnt.ksft_error; }
 
 static inline void ksft_print_header(void)
 {
+	/*
+	 * Force line buffering; If stdout is not connected to a terminal, it
+	 * will otherwise default to fully buffered, which can cause output
+	 * duplication if there is content in the buffer when fork()ing. If
+	 * there is a crash, line buffering also means the most recent output
+	 * line will be visible.
+	 */
+	setvbuf(stdout, NULL, _IOLBF, 0);
+
 	if (!(getenv("KSFT_TAP_LEVEL")))
 		printf("TAP version 13\n");
 }
diff --git a/tools/testing/selftests/kselftest/runner.sh b/tools/testing/selftests/kselftest/runner.sh
index 1c952d1401d4..261c73cab41b 100644
--- a/tools/testing/selftests/kselftest/runner.sh
+++ b/tools/testing/selftests/kselftest/runner.sh
@@ -105,15 +105,18 @@ run_one()
 		echo "# Warning: file $TEST is missing!"
 		echo "not ok $test_num $TEST_HDR_MSG"
 	else
+		if [ -x /usr/bin/stdbuf ]; then
+			stdbuf="/usr/bin/stdbuf --output=L "
+		fi
 		eval kselftest_cmd_args="\$${kselftest_cmd_args_ref:-}"
-		cmd="./$BASENAME_TEST $kselftest_cmd_args"
+		cmd="$stdbuf ./$BASENAME_TEST $kselftest_cmd_args"
 		if [ ! -x "$TEST" ]; then
 			echo "# Warning: file $TEST is not executable"
 
 			if [ $(head -n 1 "$TEST" | cut -c -2) = "#!" ]
 			then
 				interpreter=$(head -n 1 "$TEST" | cut -c 3-)
-				cmd="$interpreter ./$BASENAME_TEST"
+				cmd="$stdbuf $interpreter ./$BASENAME_TEST"
 			else
 				echo "not ok $test_num $TEST_HDR_MSG"
 				return
-- 
2.25.1