linux-kernel - Re: [PATCH] mips: micro-optimize calculate_cpu_foreign

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Date:   Thu, 30 Jun 2022 19:59:04 -0700
From:   Yury Norov <yury.norov@...il.com>
To:     Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Alexander Lobakin <alobakin@...me>,
        Huacai Chen <chenhuacai@...nel.org>,
        Jiaxun Yang <jiaxun.yang@...goat.com>,
        Mao Bibo <maobibo@...ngson.cn>,
        Philippe Mathieu-Daudé <f4bug@...at.org>,
        WANG Xuerui <kernel@...0n.name>, linux-kernel@...r.kernel.org,
        linux-mips@...r.kernel.org
Subject: Re: [PATCH] mips: micro-optimize calculate_cpu_foreign_map()

On Tue, Jun 21, 2022 at 07:47:29AM -0700, Yury Norov wrote:
> The inner loop in calculate_cpu_foreign_map() walks the whole
> cpumask to check if we have siblings for a given cpu.
> 
> We can just break after a 1st match and avoid useless traversing
> the rest of mask.
> 
> Loongarch has an identical function, so fix it here as well.
> 
> Signed-off-by: Yury Norov <yury.norov@...il.com>

Ping?

> ---
> 
> It looks like loongarch copied quite a lot from arch/mips/kernel/smp.c.
> For loongarch folks: Guys, can you consider moving shared code into a
> shared file(s)?
> 
>  arch/loongarch/kernel/smp.c | 7 +++----
>  arch/mips/kernel/smp.c      | 7 +++----
>  2 files changed, 6 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/loongarch/kernel/smp.c b/arch/loongarch/kernel/smp.c
> index b8c53b755a25..c1c91df3c8ac 100644
> --- a/arch/loongarch/kernel/smp.c
> +++ b/arch/loongarch/kernel/smp.c
> @@ -463,17 +463,16 @@ static inline void set_cpu_core_map(int cpu)
>   */
>  void calculate_cpu_foreign_map(void)
>  {
> -	int i, k, core_present;
> +	int i, k;
>  	cpumask_t temp_foreign_map;
>  
>  	/* Re-calculate the mask */
>  	cpumask_clear(&temp_foreign_map);
>  	for_each_online_cpu(i) {
> -		core_present = 0;
>  		for_each_cpu(k, &temp_foreign_map)
>  			if (cpus_are_siblings(i, k))
> -				core_present = 1;
> -		if (!core_present)
> +				break;
> +		if (k >= nr_cpu_ids)
>  			cpumask_set_cpu(i, &temp_foreign_map);
>  	}
>  
> diff --git a/arch/mips/kernel/smp.c b/arch/mips/kernel/smp.c
> index 1d93b85271ba..a2ce641f5f18 100644
> --- a/arch/mips/kernel/smp.c
> +++ b/arch/mips/kernel/smp.c
> @@ -115,17 +115,16 @@ static inline void set_cpu_core_map(int cpu)
>   */
>  void calculate_cpu_foreign_map(void)
>  {
> -	int i, k, core_present;
> +	int i, k;
>  	cpumask_t temp_foreign_map;
>  
>  	/* Re-calculate the mask */
>  	cpumask_clear(&temp_foreign_map);
>  	for_each_online_cpu(i) {
> -		core_present = 0;
>  		for_each_cpu(k, &temp_foreign_map)
>  			if (cpus_are_siblings(i, k))
> -				core_present = 1;
> -		if (!core_present)
> +				break;
> +		if (k >= nr_cpu_ids)
>  			cpumask_set_cpu(i, &temp_foreign_map);
>  	}
>  
> -- 
> 2.32.0