[PATCH 5/5] cpumask: fix comment of cpumask_xxx

[Vernon Yang]

On Mon, Mar 06, 2023 at 09:29:10AM -0800, Linus Torvalds wrote:
> On Mon, Mar 6, 2023 at 8:07 AM Vernon Yang <vernon2gm at gmail.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.

I also just see nr_cpumask_size exposed to outside, so... Sorry.

>
> 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.

Yes, cause is the cpumask_next() calls find_next_bit(..., size, ...), and
find_next_bit(..., size, ...) if no bits are set, returns @size.

@size was a nr_cpumask_bits variable before, now it is small_cpumask_bits, and
when NR_CPUS < = BITS_PER_LONG, small_cpumask_bits is a macro, which is
replaced with NR_CPUS at compile, so only the NR_CPUS is returned when it no
further cpus set.

But before nr_cpumask_bits variable, it was read while running, and it was
mutable.

The random.c try_to_generate_entropy() to get first cpu by
`if (cpu == nr_cpumask_bits)`, but cpumask_next() alway return NR_CPUS,
nr_cpumask_bits is nr_cpu_ids, so pass NR_CPUS to add_timer_on(),

>
>                 Linus
>
>                 Linus