Subject: Re: [PATCH 07/16] smp: optimize smp_call_function_many_cond()

From: Peter Zijlstra <[email-masked]>
Date: Mon, 18 Jul 2022 23:26:48 +0200

On Mon, Jul 18, 2022 at 12:28:35PM -0700, Yury Norov wrote:

> diff --git a/kernel/smp.c b/kernel/smp.c
> index dd215f439426..7ed2b9b12f74 100644
> --- a/kernel/smp.c
> +++ b/kernel/smp.c
> @@ -880,6 +880,28 @@ EXPORT_SYMBOL_GPL(smp_call_function_any);
>  #define SCF_WAIT	(1U << 0)
>  #define SCF_RUN_LOCAL	(1U << 1)
>  
> +/* Check if we need remote execution, i.e., any CPU excluding this one. */
> +static inline bool __need_remote_exec(const struct cpumask *mask, unsigned int this_cpu)
> +{
> +	unsigned int cpu;
> +
> +	switch (num_online_cpus()) {
> +	case 0:
> +		return false;
> +	case 1:
> +		return cpu_online(this_cpu) ? false : true;
> +	default:
> +		if (mask == cpu_online_mask)
> +			return true;
> +	}
> +
> +	cpu = cpumask_first_and(mask, cpu_online_mask);
> +	if (cpu == this_cpu)
> +		cpu = cpumask_next_and(cpu, mask, cpu_online_mask);
> +
> +	return cpu < nr_cpu_ids;
> +}
> +
>  static void smp_call_function_many_cond(const struct cpumask *mask,
>  					smp_call_func_t func, void *info,
>  					unsigned int scf_flags,
> @@ -916,12 +938,7 @@ static void smp_call_function_many_cond(const struct cpumask *mask,
>  	if ((scf_flags & SCF_RUN_LOCAL) && cpumask_test_cpu(this_cpu, mask))
>  		run_local = true;
>  
> -	/* Check if we need remote execution, i.e., any CPU excluding this one. */
> -	cpu = cpumask_first_and(mask, cpu_online_mask);
> -	if (cpu == this_cpu)
> -		cpu = cpumask_next_and(cpu, mask, cpu_online_mask);
> -	if (cpu < nr_cpu_ids)
> -		run_remote = true;
> +	run_remote = __need_remote_exec(mask, this_cpu);
>  
>  	if (run_remote) {
>  		cfd = this_cpu_ptr(&cfd_data);

This is more complex code for, very little to no gain. Why ?!