linux-kernel - Re: [PATCH v4 0/8] support for bitmap (and hence CPU) list "N" abbreviation

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Date:   Wed, 10 Feb 2021 15:50:07 -0800
From:   Yury Norov <yury.norov@...il.com>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Paul Gortmaker <paul.gortmaker@...driver.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Li Zefan <lizefan@...wei.com>, Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Josh Triplett <josh@...htriplett.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>
Subject: Re: [PATCH v4 0/8] support for bitmap (and hence CPU) list "N" abbreviation

On Wed, Feb 10, 2021 at 9:57 AM Paul E. McKenney <paulmck@...nel.org> wrote:
>
> On Wed, Feb 10, 2021 at 06:26:54PM +0200, Andy Shevchenko wrote:
> > On Tue, Feb 09, 2021 at 05:58:59PM -0500, Paul Gortmaker wrote:
> > > The basic objective here was to add support for "nohz_full=8-N" and/or
> > > "rcu_nocbs="4-N" -- essentially introduce "N" as a portable reference
> > > to the last core, evaluated at boot for anything using a CPU list.
> >
> > I thought we kinda agreed that N is confusing and L is better.
> > N to me is equal to 32 on 32 core system as *number of cores / CPUs*. While L
> > sounds better as *last available CPU number*.
>
> The advantage of "N" is that people will automatically recognize it as
> "last thing" or number of things" because "N" has long been used in
> both senses.  In contrast, someone seeing "0-L" for the first time is
> likely to go "What???".
>
> Besides, why would someone interpret "N" as "number of CPUs" when doing
> that almost always gets you an invalid CPU number?
>
>                                                         Thanx, Paul

I have no strong opinion about a letter, but I like Andy's idea to make it
case-insensitive.

There is another comment from the previous iteration not addressed so far.

This idea of the N notation is to make the bitmap list interface more robust
when we share the configs between different machines. What we have now
is definitely a good thing, but not completely portable except for cases
'N', '0-N' and 'N-N'.

For example, if one user adds rcu_nocbs= '4-N', and it works perfectly fine for
him, another user with s NR_CPUS == 2 will fail to boot with such a config.

This is not a problem of course in case of absolute values because nobody
guaranteed robustness. But this N feature would be barely useful in practice,
except for 'N', '0-N' and 'N-N' as I mentioned before, because there's always
a chance to end up with a broken config.

We can improve on robustness a lot if we take care about this case.For me,
the more reliable interface would look like this:
1. chunks without N work as before.
2. if 'a-N' is passed where a>=N, we drop chunk and print warning message
3. if 'a-N' is passed where a>=N together with a control key, we set last bit
and print warning.

For example, on 2-core CPU:
"4-2" --> error
"4-4" --> error
"4-N" --> drop and warn
"X, 4-N" --> set last bit and warn

Any comments?