Re: [PATCH 8/8] rcu: deprecate "all" option to rcu_nocbs=

[Yury Norov <yury.norov@gmail.com>]

On Tue, Jan 26, 2021 at 9:12 AM Paul Gortmaker
<paul.gortmaker@windriver.com> wrote:
>
> With the core bitmap support now accepting "N" as a placeholder for
> the end of the bitmap, "all" can be represented as "0-N" and has the
> advantage of not being specific to RCU (or any other subsystem).
>
> So deprecate the use of "all" by removing documentation references
> to it.  The support itself needs to remain for now, since we don't
> know how many people out there are using it currently, but since it
> is in an __init area anyway, it isn't worth losing sleep over.
>
> Cc: Yury Norov <yury.norov@gmail.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: "Paul E. McKenney" <paulmck@kernel.org>
> Cc: Josh Triplett <josh@joshtriplett.org>
> Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
> ---
>  Documentation/admin-guide/kernel-parameters.txt | 4 +---
>  kernel/rcu/tree_plugin.h                        | 6 ++----
>  2 files changed, 3 insertions(+), 7 deletions(-)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index a10b545c2070..a116c0ff0a91 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -4037,9 +4037,7 @@
>                                 see CONFIG_RAS_CEC help text.
>
>         rcu_nocbs=      [KNL]
> -                       The argument is a cpu list, as described above,
> -                       except that the string "all" can be used to
> -                       specify every CPU on the system.
> +                       The argument is a cpu list, as described above.
>
>                         In kernels built with CONFIG_RCU_NOCB_CPU=y, set
>                         the specified list of CPUs to be no-callback CPUs.
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index 7e291ce0a1d6..56788dfde922 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -1463,14 +1463,12 @@ static void rcu_cleanup_after_idle(void)
>
>  /*
>   * Parse the boot-time rcu_nocb_mask CPU list from the kernel parameters.
> - * The string after the "rcu_nocbs=" is either "all" for all CPUs, or a
> - * comma-separated list of CPUs and/or CPU ranges.  If an invalid list is
> - * given, a warning is emitted and all CPUs are offloaded.
> + * If the list is invalid, a warning is emitted and all CPUs are offloaded.
>   */
>  static int __init rcu_nocb_setup(char *str)
>  {
>         alloc_bootmem_cpumask_var(&rcu_nocb_mask);
> -       if (!strcasecmp(str, "all"))
> +       if (!strcasecmp(str, "all"))            /* legacy: use "0-N" instead */

I think 'all' and 'none' is a good idea. It's simple and convenient.
But if you don't
like it, can you please at least put this comment in system log using
WARN_ON_ONCE(). It's quite possible that Linux users don't read source code
comments.

>                 cpumask_setall(rcu_nocb_mask);
>         else
>                 if (cpulist_parse(str, rcu_nocb_mask)) {
> --
> 2.17.1
>