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Subject: Re: [PATCH v4 0/8] support for bitmap (and hence CPU) list "N"
 abbreviation
To:     paulmck@kernel.org,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc:     Paul Gortmaker <paul.gortmaker@windriver.com>,
        linux-kernel@vger.kernel.org, Li Zefan <lizefan@huawei.com>,
        Ingo Molnar <mingo@kernel.org>,
        Yury Norov <yury.norov@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Josh Triplett <josh@joshtriplett.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Frederic Weisbecker <fweisbec@gmail.com>
References: <20210209225907.78405-1-paul.gortmaker@windriver.com>
 <YCQJToq1d63BU55S@smile.fi.intel.com>
 <20210210175751.GH2743@paulmck-ThinkPad-P72>
From:   Rasmus Villemoes <linux@rasmusvillemoes.dk>
Message-ID: <da9ca86e-4113-f0ad-6120-3b3cbea5c836@rasmusvillemoes.dk>
Date:   Thu, 11 Feb 2021 12:04:51 +0100
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On 10/02/2021 18.57, Paul E. McKenney 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???".

Completely agree. The patch that introduces this even updates
Documentation/ at the same time, and if people are confused just because
they don't RTFM, xkcd#293 applies. So let's please just paint the
bikeshed N. (As for case insensitivity, I don't see the point, it just
makes documentation and implementation more cumbersome and confusing.
Just document and implement _one_ way of doing this.)

As for a future syntax for "last 4 cpus", it's common to accept a
negative index to mean count from the end, so unless we already accept
-4 as a shorthand for 0-4 (haven't checked), that could be -4-N. But
regardless, I also agree with Paul on this point, that's for a future
time when the need arises.

Rasmus