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

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Message-ID: <xhsmhv8q0bj6m.mognet@vschneid.remote.csb>
Date:   Tue, 06 Sep 2022 16:45:37 +0100
From:   Valentin Schneider <vschneid@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Yury Norov <yury.norov@...il.com>, 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 16:38, Peter Zijlstra wrote:
> On Tue, Sep 06, 2022 at 01:06:47PM +0100, Valentin Schneider wrote:
>
>>   #define nr_cpumask_bits nr_cpu_ids
>
> That assumes the CPU space is dense; is this so? That is, you can have 4
> CPUs while the highest cpu number is vastly larger than 4.
>
> It's uncommon, but not unheard of I think. ISTR some BIOSes leaving
> holes in the CPU space when there were empty CPU sockets on the
> motherboard.

I'd assume this would be visible in the cpu_possible_mask and thus be
properly reflected in nr_cpu_ids. Otherwise that would already break with
CONFIG_CPUMASK_OFFSTACK=y, I think.