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From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To: Chuyi Zhou <zhouchuyi@bytedance.com>
Cc: tglx@linutronix.de, mingo@redhat.com, luto@kernel.org,
	peterz@infradead.org, paulmck@kernel.org, muchun.song@linux.dev,
	bp@alien8.de, dave.hansen@linux.intel.com, pbonzini@redhat.com,
	clrkwllms@kernel.org, rostedt@goodmis.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3 06/12] smp: Enable preemption early in
 smp_call_function_many_cond
Message-ID: <20260318165503.KTk9t8lK@linutronix.de>
References: <20260318045638.1572777-1-zhouchuyi@bytedance.com>
 <20260318045638.1572777-7-zhouchuyi@bytedance.com>
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In-Reply-To: <20260318045638.1572777-7-zhouchuyi@bytedance.com>

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