Re: [PATCH 3/9] lib/group_cpus: relax atomicity requirement in grp_spread_init_one()

[Yury Norov]

On Fri, Dec 29, 2023 at 10:39 AM Andrew Morton
<akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Thu, 28 Dec 2023 12:09:30 -0800 Yury Norov <yury.norov@xxxxxxxxx> wrote:
>
> > Because nmsk and irqmsk are stable, extra atomicity is not required.
> >
> > Signed-off-by: Yury Norov <yury.norov@xxxxxxxxx>
> > NAKed-by: Ming Lei <ming.lei@xxxxxxxxxx>
>
> Well that's unusual.  I suggest that the changelog at least describe the
> objection, and its counterargument?

Sorry, forgot to copy it from v3 discussion. Please find below:


> > > > I think this kind of change should be avoided, here the code is
> > > > absolutely in slow path, and we care code cleanness and readability
> > > > much more than the saved cycle from non atomicity.
> > >
> > > Atomic ops have special meaning and special function. This 'atomic' way
> > > of moving a bit from one bitmap to another looks completely non-trivial
> > > and puzzling to me.
> > >
> > > A sequence of atomic ops is not atomic itself. Normally it's a sing of
> > > a bug. But in this case, both masks are stable, and we don't need
> > > atomicity at all.
> >
> > Here we don't care the atomicity.
> >
> > >
> > > It's not about performance, it's about readability.
> >
> > __cpumask_clear_cpu() and __cpumask_set_cpu() are more like private
> > helper, and more hard to follow.
>
> No that's not true. Non-atomic version of the function is not a
> private helper of course.
>
> > [@linux]$ git grep -n -w -E "cpumask_clear_cpu|cpumask_set_cpu" ./ | wc
> >     674    2055   53954
> > [@linux]$ git grep -n -w -E "__cpumask_clear_cpu|__cpumask_set_cpu" ./ | wc
> >      21      74    1580
> >
> > I don't object to comment the current usage, but NAK for this change.
>
> No problem, I'll add you NAK.

You can add the following words meantime:

__cpumask_clear_cpu() and __cpumask_set_cpu() are added in commit 6c8557bdb28d
("smp, cpumask: Use non-atomic cpumask_{set,clear}_cpu()") for fast code path(
smp_call_function_many()).

We have ~670 users of cpumask_clear_cpu & cpumask_set_cpu, lots of them
fall into same category with group_cpus.c(doesn't care atomicity, not in fast
code path), and needn't change to __cpumask_clear_cpu() and __cpumask_set_cpu().
Otherwise, this way may encourage to update others into the __cpumask_* version.