netdev - Re: [PATCH 5/5] cpumask: fix comment of cpumask

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Date:   Mon, 6 Mar 2023 09:29:10 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Vernon Yang <vernon2gm@...il.com>
Cc:     tytso@....edu, Jason@...c4.com, davem@...emloft.net,
        edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com,
        jejb@...ux.ibm.com, martin.petersen@...cle.com,
        yury.norov@...il.com, andriy.shevchenko@...ux.intel.com,
        linux@...musvillemoes.dk, james.smart@...adcom.com,
        dick.kennedy@...adcom.com, linux-kernel@...r.kernel.org,
        wireguard@...ts.zx2c4.com, netdev@...r.kernel.org,
        linux-scsi@...r.kernel.org
Subject: Re: [PATCH 5/5] cpumask: fix comment of cpumask_xxx

On Mon, Mar 6, 2023 at 8:07 AM Vernon Yang <vernon2gm@...il.com> wrote:
>
> After commit 596ff4a09b89 ("cpumask: re-introduce constant-sized cpumask
> optimizations"), the cpumask size is divided into three different case,
> so fix comment of cpumask_xxx correctly.

No no.

Those three cases are meant to be entirely internal optimizations.
They are literally just "preferred sizes".

The correct thing to do is always that

   * Returns >= nr_cpu_ids if no cpus set.

because nr_cpu_ids is always the *smallest* of the access sizes.

That's exactly why it's a ">=". The CPU mask stuff has always
historically potentially used a different size than the actual
nr_cpu_ids, in that it could do word-sized scans even when the machine
might only have a smaller set of CPUs.

So the whole "small" vs "large" should be seen entirely internal to
cpumask.h. We should not expose it outside (sadly, that already
happened with "nr_cpumask_size", which also was that kind of thing.

So no, this patch is wrong. If anything, the comments should be strengthened.

Of course, right now Guenter seems to be reporting a problem with that
optimization, so unless I figure out what is going on I'll just need
to revert it anyway.

                Linus

                Linus