linux-kernel - Re: [PATCH v1 1/9] selftests: Line buffer test program's stdout

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Date:   Thu, 13 Jul 2023 15:16:24 +0100
From:   Mark Brown <broonie@...nel.org>
To:     Ryan Roberts <ryan.roberts@....com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Shuah Khan <shuah@...nel.org>,
        Jérôme Glisse <jglisse@...hat.com>,
        David Hildenbrand <david@...hat.com>,
        John Hubbard <jhubbard@...dia.com>,
        Florent Revest <revest@...omium.org>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v1 1/9] selftests: Line buffer test program's stdout

On Thu, Jul 13, 2023 at 02:54:32PM +0100, Ryan Roberts wrote:
> 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().

This is going to be useful in general since not all selftests use the
kselftest helpers but it'd probably also be good to make
ksft_print_header() also make the output unbuffered so that if setbuf
isn't installed on the target system or the tests are run standalone we
don't run into issues there.  Even if the test isn't corrupting data
having things unbuffered is going to be good for making sure we don't
drop any output if the test dies.

> +		if [ -x /usr/bin/stdbuf ]; then
> +			stdbuf="/usr/bin/stdbuf --output=L "
> +		fi

Might be more robust to use type -p to find stdbuf in case it's in /bin
or something?

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