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

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Date:   Thu, 11 Feb 2021 16:23:39 -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 04:23:09PM -0800, Paul E. McKenney wrote:
> On Wed, Feb 10, 2021 at 03:50:07PM -0800, Yury Norov wrote:
> > 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?
> 
> We really don't know the user's intent, and we cannot have complete
> portability without knowing the user's intent.  For example, "4-N" means
> "all but the first four CPUs", in which case an error is appropriate
> because "4-N" makes no more sense on a 2-CPU system than does "4-1".
> I could see a potential desire for some notation for "the last two CPUs",
> but let's please have a real need for such a thing before overengineering
> this patch series any further.
> 
> To get the level of portability you seem to be looking for, we need some
> higher-level automation that knows how many CPUs there are and what
> the intent is.  That automation can then generate the cpumasks for a
> given system.  But for more typical situations, what Paul has now will
> work fine.
> 
> Paul Gortmaker's patch series is doing something useful.  We should
> not let potential future desires prevent us from taking a very useful
> step forward.
> 
> 							Thanx, Paul

No problem, we can do it later if it will become a real concern. 

Can you please remove this series from linux-next unless we finish
the review? It prevents me from applying the series from the LKML.

Yury