linux-kernel - Re: [PATCH RFC cpumask 3/5] cpumask: Add a "none" alias to complement "all"

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Date:   Tue, 5 Jan 2021 22:59:52 -0800
From:   Yury Norov <yury.norov@...il.com>
To:     paulmck@...nel.org
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>, kernel-team@...com,
        Paul Gortmaker <paul.gortmaker@...driver.com>
Subject: Re: [PATCH RFC cpumask 3/5] cpumask: Add a "none" alias to complement "all"

On Tue, Jan 5, 2021 at 4:49 PM <paulmck@...nel.org> wrote:
>
> From: Paul Gortmaker <paul.gortmaker@...driver.com>
>
> With global support for a CPU list alias of "all", it seems to just make
> sense to also trivially extend support for an opposite "none" specifier.
>
> Signed-off-by: Paul Gortmaker <paul.gortmaker@...driver.com>
> Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> ---
>  Documentation/admin-guide/kernel-parameters.rst | 6 ++++++
>  lib/cpumask.c                                   | 5 +++++
>  2 files changed, 11 insertions(+)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.rst b/Documentation/admin-guide/kernel-parameters.rst
> index cdf4e81..7dd1224 100644
> --- a/Documentation/admin-guide/kernel-parameters.rst
> +++ b/Documentation/admin-guide/kernel-parameters.rst
> @@ -76,6 +76,12 @@ is equivalent to "foo_cpus=0-N" -- where "N" is the numerically last CPU on
>  the system, thus avoiding looking up the value in "/sys/devices/system/cpu"
>  in advance on each deployed system.
>
> +        foo_cpus=none
> +
> +will provide an empty/cleared cpu mask for the associated boot argument.
> +
> +Note that "all" and "none" are not necessarily valid/sensible input values
> +for each available parameter expecting a CPU list.
>
>  This document may not be entirely up to date and comprehensive. The command
>  "modinfo -p ${modulename}" shows a current list of all parameters of a loadable
> diff --git a/lib/cpumask.c b/lib/cpumask.c
> index 9f8ff72..7fbcab8 100644
> --- a/lib/cpumask.c
> +++ b/lib/cpumask.c
> @@ -110,6 +110,11 @@ int cpulist_parse(const char *buf, struct cpumask *dstp)
>                 return 0;
>         }
>
> +       if (!strcmp(buf, "none")) {
> +               cpumask_clear(dstp);
> +               return 0;
> +       }

Same comment as to the patch 2. Also, what if a user wants to stack ranges
like 'all, 1-3, none'? As far as I understand current implementation,
cpu_parselist
will pass new keywords to bitmap_parselist() which will fail to parse it.

If you think of new extensions as special case keywords which should not be
mixed with traditional region descriptors, I'm OK with that. But it should be
explained in documentation. I think it's better to handle 'all' and 'none' in
bitmap_parse_region().

The parselist() supports partially used groups with the notation like:
0-1023:2/256 ==> 0,1,256,257,512,513,768,769
I think it's worth making sure that new keywords work with groups smoothly,
otherwise mention in documentation that they don't.

>         return bitmap_parselist(buf, cpumask_bits(dstp), nr_cpumask_bits);
>  }
>  EXPORT_SYMBOL(cpulist_parse);
> --
> 2.9.5
>