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

Peter Zijlstra
2022-07-18T21:28:30Z

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 ?!