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Date:   Tue, 12 Dec 2023 08:52:14 -0800
From:   Yury Norov <yury.norov@gmail.com>
To:     Ming Lei <ming.lei@redhat.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        linux-kernel@vger.kernel.org,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>
Subject: Re: [PATCH v3 3/7] lib/group_cpus: relax atomicity requirement in
 grp_spread_init_one()
Message-ID: <ZXiPvgzZvXyWfarS@yury-ThinkPad>
References: <20231212042108.682072-1-yury.norov@gmail.com>
 <20231212042108.682072-4-yury.norov@gmail.com>
 <ZXgszD9tIKY1tC9r@fedora>
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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.

It's not about performance, it's about readability.

Thanks,
Yury