linux-kernel - Re: [PATCH 03/11] smp: Remove get_cpu from smp_call_function

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Message-ID: <20260205094226.GF232055@noisy.programming.kicks-ass.net>
Date: Thu, 5 Feb 2026 10:42:26 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Chuyi Zhou <zhouchuyi@...edance.com>
Cc: tglx@...utronix.de, mingo@...hat.com, luto@...nel.org,
	paulmck@...nel.org, muchun.song@...ux.dev, bp@...en8.de,
	dave.hansen@...ux.intel.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 03/11] smp: Remove get_cpu from smp_call_function_any

On Tue, Feb 03, 2026 at 07:23:53PM +0800, Chuyi Zhou wrote:
> Now smp_call_function_single() would enable preemption before
> csd_lock_wait() to reduce the critical section. To allow callers of
> smp_call_function_any() to also benefit from this optimization, remove
> get_cpu()/put_cpu() from smp_call_function_any().
> 
> Signed-off-by: Chuyi Zhou <zhouchuyi@...edance.com>
> ---
>  kernel/smp.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/smp.c b/kernel/smp.c
> index 0858553f3666..f572716c3c7d 100644
> --- a/kernel/smp.c
> +++ b/kernel/smp.c
> @@ -772,13 +772,18 @@ int smp_call_function_any(const struct cpumask *mask,
>  	unsigned int cpu;
>  	int ret;
>  
> +	/*
> +	 * Prevent migration to another CPU after selecting the current CPU
> +	 * as the target.
> +	 */
> +	guard(migrate)();
> +
>  	/* Try for same CPU (cheapest) */
> -	cpu = get_cpu();
> +	cpu = smp_processor_id();
>  	if (!cpumask_test_cpu(cpu, mask))
>  		cpu = sched_numa_find_nth_cpu(mask, 0, cpu_to_node(cpu));
>  
>  	ret = smp_call_function_single(cpu, func, info, wait);
> -	put_cpu();
>  	return ret;

Urgh, that's horrible.

Basically what you want is something like so:

	bool enable = true;
	unsigned int cpu;
	int ret;

	preempt_disable();
	cpu = smp_processor_id();
	if (!cpumask_test_cpu(cpu, mask)) {
		cpu = sched_numa_find_nth_cpu(mask, 0, cpu_to_node(cpu))
		enable = false;
		preempt_enable();
	}

	ret = smp_call_function_single(cpu, func, info, wait);
	if (enable)
		preempt_enable();
	return ret;