linux-kernel - Re: [PATCH v3 06/10] selftests/mm: Don't fail uffd-stress if too many CPUs

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Message-ID: <CA+i-1C2pn90pfB6qhuGV8ecUw3YKfg1b8tSTsXs4n_Rw6US9mA@mail.gmail.com>
Date: Wed, 5 Mar 2025 12:07:14 +0100
From: Brendan Jackman <jackmanb@...gle.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, Andrew Morton <akpm@...ux-foundation.org>, 
	Shuah Khan <shuah@...nel.org>
Cc: Dev Jain <dev.jain@....com>, linux-mm@...ck.org, linux-kselftest@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Mateusz Guzik <mjguzik@...il.com>
Subject: Re: [PATCH v3 06/10] selftests/mm: Don't fail uffd-stress if too many CPUs

On Fri, 28 Feb 2025 at 17:55, Brendan Jackman <jackmanb@...gle.com> wrote:
>
> This calculation divides a fixed parameter by an environment-dependent
> parameter i.e. the number of CPUs.
>
> The simple way to avoid machine-specific failures here is to just put a
> cap on the max value of the latter.
>
> Suggested-by: Mateusz Guzik <mjguzik@...il.com>
> Signed-off-by: Brendan Jackman <jackmanb@...gle.com>
> ---
>  tools/testing/selftests/mm/uffd-stress.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
>
> diff --git a/tools/testing/selftests/mm/uffd-stress.c b/tools/testing/selftests/mm/uffd-stress.c
> index efe2051c393096e237d942c04a264b6611a6e127..5656128590373ed376b3b5d9259e5ca3867a4099 100644
> --- a/tools/testing/selftests/mm/uffd-stress.c
> +++ b/tools/testing/selftests/mm/uffd-stress.c
> @@ -434,6 +434,7 @@ static void sigalrm(int sig)
>
>  int main(int argc, char **argv)
>  {
> +       unsigned long nr_cpus;
>         size_t bytes;
>
>         if (argc < 4)
> @@ -452,7 +453,15 @@ int main(int argc, char **argv)
>                 return KSFT_SKIP;
>         }
>
> -       nr_threads = sysconf(_SC_NPROCESSORS_ONLN);
> +       nr_cpus = sysconf(_SC_NPROCESSORS_ONLN);
> +       if (nr_cpus > 32) {
> +               /* Don't let calculation below go to zero. */
> +               ksft_print_msg("_SC_NPROCESSORS_ONLN (%lu) too large, capping nr_threads to 32\n",
> +                              nr_cpus);
> +               nr_threads = 32;
> +       } else {
> +               nr_cpus = nr_threads;

Won't have time to send v4 for a few days so I'll just note here: this
shoudl be nr_thread = nr_cpus. This causes a division by zero on
machines with less than 30 CPUs.