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

From: "Paul E. McKenney" <paulmck@kernel.org>
To: Yury Norov <yury.norov@gmail.com>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
	Paul Gortmaker <paul.gortmaker@windriver.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Li Zefan <lizefan@huawei.com>, Ingo Molnar <mingo@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Josh Triplett <josh@joshtriplett.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Frederic Weisbecker <fweisbec@gmail.com>,
	Rasmus Villemoes <linux@rasmusvillemoes.dk>
Subject: Re: [PATCH v4 0/8] support for bitmap (and hence CPU) list "N" abbreviation
Date: Thu, 11 Feb 2021 16:38:22 -0800	[thread overview]
Message-ID: <20210212003822.GN2743@paulmck-ThinkPad-P72> (raw)
In-Reply-To: <20210212002339.GA167389@yury-ThinkPad>

On Thu, Feb 11, 2021 at 04:23:39PM -0800, Yury Norov wrote:
> 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@kernel.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.

That will happen shortly, but in the meantime, just do the following on
top of -next before applying Paul's latest series:

	git revert b3c314b ed78166 1e792c4 e831c73

							Thanx, Paul