linux-kernel - Re: [PATCH v3 2/7] lib/group_cpus: optimize inner loop in grp_spread_init

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Date:   Tue, 12 Dec 2023 09:04:19 -0800
From:   Yury Norov <yury.norov@...il.com>
To:     Ming Lei <ming.lei@...hat.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 2/7] lib/group_cpus: optimize inner loop in
 grp_spread_init_one()

On Tue, Dec 12, 2023 at 05:46:53PM +0800, Ming Lei wrote:
> On Mon, Dec 11, 2023 at 08:21:02PM -0800, Yury Norov wrote:
> > The loop starts from the beginning every time we switch to the next
> > sibling mask. This is the Schlemiel the Painter's style of coding
> > because we know for sure that nmsk is clear up to current CPU, and we
> > can just continue from the next CPU.
> > 
> > Also, we can do it nicer if leverage the dedicated for_each() iterator,
> > and simplify the logic of clearing a bit in nmsk.
> > 
> > Signed-off-by: Yury Norov <yury.norov@...il.com>
> > ---
> >  lib/group_cpus.c | 13 ++++++-------
> >  1 file changed, 6 insertions(+), 7 deletions(-)
> > 
> > diff --git a/lib/group_cpus.c b/lib/group_cpus.c
> > index ee272c4cefcc..10dead3ab0e0 100644
> > --- a/lib/group_cpus.c
> > +++ b/lib/group_cpus.c
> > @@ -30,14 +30,13 @@ static void grp_spread_init_one(struct cpumask *irqmsk, struct cpumask *nmsk,
> >  
> >  		/* If the cpu has siblings, use them first */
> >  		siblmsk = topology_sibling_cpumask(cpu);
> > -		for (sibl = -1; cpus_per_grp > 0; ) {
> > -			sibl = cpumask_next(sibl, siblmsk);
> > -			if (sibl >= nr_cpu_ids)
> > -				break;
> > -			if (!cpumask_test_and_clear_cpu(sibl, nmsk))
> > -				continue;
> > +		sibl = cpu + 1;
> 
> It doesn't have to 'cpu + 1', see below comment.
> 
> > +
> > +		for_each_cpu_and_from(sibl, siblmsk, nmsk) {
> > +			cpumask_clear_cpu(sibl, nmsk);
> >  			cpumask_set_cpu(sibl, irqmsk);
> > -			cpus_per_grp--;
> > +			if (cpus_per_grp-- == 0)
> 
> 			if (--cpus_per_grp == 0)
 
That's right, I'll send a new version this weekend.

> > +				return;
> 
> I think for_each_cpu_and() should work just fine, cause cpu has been cleared
> from nmsk, so the change can be something like, then patch 1 isn't
> necessary.
 
It works just fine except that it's O(N^2), where O(N) is easily
achievable. Again, it's not about performance, it's about coding
habits.
 
> 		for_each_cpu_and(sibl, siblmsk, nmsk) {
> 			cpumask_clear_cpu(sibl, nmsk);
>   			cpumask_set_cpu(sibl, irqmsk);
> 			if (--cpus_per_grp == 0)
> 				return;
> 		}
> 
> 
> Thanks,
> Ming