Re: [PATCH v3 3/7] lib/group_cpus: relax atomicity requirement in grp_spread_init_one()

Ming Lei
2023-12-13T00:16:13Z

On Tue, Dec 12, 2023 at 08:52:14AM -0800, Yury Norov wrote:
> On Tue, Dec 12, 2023 at 05:50:04PM +0800, Ming Lei wrote:
> > On Mon, Dec 11, 2023 at 08:21:03PM -0800, Yury Norov wrote:
> > > Because nmsk and irqmsk are stable, extra atomicity is not required.
> > >
> > > Signed-off-by: Yury Norov <yury.norov@gmail.com>
> > > ---
> > > lib/group_cpus.c | 8 ++++----
> > > 1 file changed, 4 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/lib/group_cpus.c b/lib/group_cpus.c
> > > index 10dead3ab0e0..7ac94664230f 100644
> > > --- a/lib/group_cpus.c
> > > +++ b/lib/group_cpus.c
> > > @@ -24,8 +24,8 @@ static void grp_spread_init_one(struct cpumask *irqmsk, struct cpumask *nmsk,
> > > if (cpu >= nr_cpu_ids)
> > > return;
> > >
> > > - cpumask_clear_cpu(cpu, nmsk);
> > > - cpumask_set_cpu(cpu, irqmsk);
> > > + __cpumask_clear_cpu(cpu, nmsk);
> > > + __cpumask_set_cpu(cpu, irqmsk);
> > > cpus_per_grp--;
> > >
> > > /* If the cpu has siblings, use them first */
> > > @@ -33,8 +33,8 @@ static void grp_spread_init_one(struct cpumask *irqmsk, struct cpumask *nmsk,
> > > sibl = cpu + 1;
> > >
> > > for_each_cpu_and_from(sibl, siblmsk, nmsk) {
> > > - cpumask_clear_cpu(sibl, nmsk);
> > > - cpumask_set_cpu(sibl, irqmsk);
> > > + __cpumask_clear_cpu(sibl, nmsk);
> > > + __cpumask_set_cpu(sibl, irqmsk);
> >
> > 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.

[@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.

Thanks,
Ming