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

Ming Lei
2023-12-14T00:44:13Z

On Wed, Dec 13, 2023 at 09:03:17AM -0800, Yury Norov wrote:
> On Wed, Dec 13, 2023 at 08:14:45AM +0800, Ming Lei wrote:
> > 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.
>
> 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.

Thanks,
Ming