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Date: Sat, 20 Jan 2024 15:03:00 +0800
From: Ming Lei <ming.lei@redhat.com>
To: Yury Norov <yury.norov@gmail.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>,
	Breno Leitao <leitao@debian.org>,
	Nathan Chancellor <nathan@kernel.org>,
	Rasmus Villemoes <linux@rasmusvillemoes.dk>,
	Zi Yan <ziy@nvidia.com>
Subject: Re: [PATCH 2/9] lib/group_cpus: optimize inner loop in
 grp_spread_init_one()
Message-ID: <ZatwJE6d9Kp3GuBy@fedora>
References: <20240120025053.684838-1-yury.norov@gmail.com>
 <20240120025053.684838-3-yury.norov@gmail.com>
 <Zas7LI1v5Adk/VVP@fedora>
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In-Reply-To: <Zas7LI1v5Adk/VVP@fedora>

On Sat, Jan 20, 2024 at 11:17:00AM +0800, Ming Lei wrote:
> On Fri, Jan 19, 2024 at 06:50:46PM -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@gmail.com>
> > ---
> >  lib/group_cpus.c | 14 +++++++-------
> >  1 file changed, 7 insertions(+), 7 deletions(-)
> > 
> > diff --git a/lib/group_cpus.c b/lib/group_cpus.c
> > index ee272c4cefcc..063ed9ae1b8d 100644
> > --- a/lib/group_cpus.c
> > +++ b/lib/group_cpus.c
> > @@ -30,14 +30,14 @@ 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;
> 
> No, it is silly to let 'sibl' point to 'cpu + 1', cause we just
> want to iterate over 'siblmsk & nmsk', and nothing to do with
> the next cpu('cpu + 1').
> 
> > +
> > +		for_each_cpu_and_from(sibl, siblmsk, nmsk) {
> > +			if (cpus_per_grp-- == 0)
> > +				return;
> > +
> > +			cpumask_clear_cpu(sibl, nmsk);
> >  			cpumask_set_cpu(sibl, irqmsk);
> > -			cpus_per_grp--;
> 
> Andrew, please replace the 1st two patches with the following one:
> 
> From 7a983ee5e1b4f05e5ae26c025dffd801b909e2f3 Mon Sep 17 00:00:00 2001
> From: Ming Lei <ming.lei@redhat.com>
> Date: Sat, 20 Jan 2024 11:07:26 +0800
> Subject: [PATCH] lib/group_cpus.c: simplify grp_spread_init_one()
> 
> What the inner loop needs to do is to iterate over `siblmsk & nmsk`, and
> clear the cpu in 'nmsk' and set it in 'irqmsk'.
> 
> Clean it by for_each_cpu_and().
> 
> This is based on Yury Norov's patch, which needs one extra
> for_each_cpu_and_from(), which is really not necessary.
> 
> Signed-off-by: Ming Lei <ming.lei@redhat.com>
> ---
>  lib/group_cpus.c | 11 ++++-------
>  1 file changed, 4 insertions(+), 7 deletions(-)
> 
> diff --git a/lib/group_cpus.c b/lib/group_cpus.c
> index ee272c4cefcc..564d8e817f65 100644
> --- a/lib/group_cpus.c
> +++ b/lib/group_cpus.c
> @@ -30,14 +30,11 @@ 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;
> +		for_each_cpu_and(sibl, siblmsk, nmsk) {
> +			cpumask_clear_cpu(sibl, nmsk);
>  			cpumask_set_cpu(sibl, irqmsk);
> -			cpus_per_grp--;
> +			if (--cpus_per_grp == 0)
> +				return;

Iterator variable of 'nmsk' is updated inside loop, and it is still
tricky, so please ignore it, I just sent one formal & revised patch:

https://lkml.org/lkml/2024/1/20/43


Thanks,
Ming