Re: [PATCH v3 06/12] smp: Enable preemption early in smp_call_function_many_cond

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2026-03-18 12:56:32 [+0800], Chuyi Zhou wrote:
> --- a/kernel/smp.c
> +++ b/kernel/smp.c
> @@ -907,9 +920,11 @@ static void smp_call_function_many_cond(const struct cpumask *mask,
>  		}
>  	}

So now I understand why we have this cpumask on stack.
Could we, on a preemptible kernel, where we have a preemption counter,
in the case of preemptible() allocate a cpumask and use it here? If the
allocation fails or we are not on a preemptbile kernel then we don't do
this optimized wait with enabled preemption.

There is no benefit of doing all this if the caller has already
preemption disabled.

> -	rcu_read_unlock();
> -	if (preemptible_wait)
> +	if (!preemptible_wait)
> +		put_cpu();
> +	else
>  		free_cpumask_var(cpumask_stack);
> +	rcu_read_unlock();
>  }
>  
>  /**

Sebastian