linux-kernel - Re: [PATCH v3 3/7] lib/group_cpus: relax atomicity requirement in grp_spread_init

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Date:   Thu, 14 Dec 2023 08:43:58 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Yury Norov <yury.norov@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>
Subject: Re: [PATCH v3 3/7] lib/group_cpus: relax atomicity requirement in
 grp_spread_init_one()

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@...il.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