linux-kernel - Re: [PATCH v2 0/5] cpumask: cleanup nr_cpu_ids vs nr_cpumask

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Date:   Tue, 06 Sep 2022 13:06:47 +0100
From:   Valentin Schneider <vschneid@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Yury Norov <yury.norov@...il.com>
Cc:     linux-kernel@...r.kernel.org,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Sander Vanheule <sander@...nheule.net>,
        Alexey Klimov <klimov.linux@...il.com>,
        Eric Biggers <ebiggers@...gle.com>
Subject: Re: [PATCH v2 0/5] cpumask: cleanup nr_cpu_ids vs nr_cpumask_bits mess

On 06/09/22 10:55, Peter Zijlstra wrote:
> On Mon, Sep 05, 2022 at 04:08:15PM -0700, Yury Norov wrote:
>> cpumask subsystem uses nr_cpu_ids and nr_cpumask_bits interchangeably
>> despite that the variables have different meaning and purpose. It makes
>> some cpumask functions broken.
>>
>> This series cleans that mess and adds new config FORCE_NR_CPUS that
>> allows to optimize cpumask subsystem if the number of CPUs is known
>> at compile-time.
>
> Who will use this? Distro's can't, which means 99% of people will not
> use this ever. Is it worth it?

I'd tend to agree here.

One extra thing worth noting is CONFIG_CPUMASK_OFFSTACK=n cpumask_size()
still uses NR_CPUS under the hood, despite being (mostly) used to
dynamically allocate cpumasks. So having an unconditionnal

  #define nr_cpumask_bits nr_cpu_ids

would save up some memory for those allocations.

A quick compile test on x86 defconfig (OFFSTACK=n) gives me:

  Total: Before=18711411, After=18705653, chg -0.03%

If it's in the range of barely-half-a-page on other archs, could we just
do that then?