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

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Message-Id: <ae8c9bd0-365a-473d-a0b4-99c3a15585e6@bytedance.com>
Date: Fri, 6 Feb 2026 16:54:24 +0800
From: "Chuyi Zhou" <zhouchuyi@...edance.com>
To: "Peter Zijlstra" <peterz@...radead.org>
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

在 2026/2/5 17:42, Peter Zijlstra 写道:
> 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;

OK, I will update it in the next version.