linux-kernel - Re: [PATCH 8/8] rcu: deprecate "all" option to rcu

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Date:   Tue, 26 Jan 2021 14:17:55 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Yury Norov <yury.norov@...il.com>
Cc:     Paul Gortmaker <paul.gortmaker@...driver.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        lizefan@...wei.com, Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>, josh@...htriplett.org,
        Peter Zijlstra <peterz@...radead.org>, fweisbec@...il.com,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Subject: Re: [PATCH 8/8] rcu: deprecate "all" option to rcu_nocbs=

On Tue, Jan 26, 2021 at 01:36:23PM -0800, Yury Norov wrote:
> On Tue, Jan 26, 2021 at 9:12 AM Paul Gortmaker
> <paul.gortmaker@...driver.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@...il.com>
> > Cc: Peter Zijlstra <peterz@...radead.org>
> > Cc: "Paul E. McKenney" <paulmck@...nel.org>
> > Cc: Josh Triplett <josh@...htriplett.org>
> > Signed-off-by: Paul Gortmaker <paul.gortmaker@...driver.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.

Please leave it silent.  This has been available to RCU users for
quite some time, so suddenly spewing warnings at all of them is a bit
unfriendly.  The extra code is negligible, and the documentation will
guide people more gently in the right direction.  Plus I am the one who
would end up receiving complaints about the warnings, and I have much
better things to do with my time.

							Thanx, Paul

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