linux-kernel - Re: [PATCH 4/4] smp: Defer check for local execution in smp_call_function_many

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Message-ID: <aEiAepW_zRYtC1Is@yury>
Date: Tue, 10 Jun 2025 14:59:06 -0400
From: Yury Norov <yury.norov@...il.com>
To: "Paul E. McKenney" <paulmck@...nel.org>,
	Neeraj Upadhyay <neeraj.upadhyay@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Thorsten Blum <thorsten.blum@...ux.dev>,
	Zqiang <qiang.zhang1211@...il.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Dan Carpenter <dan.carpenter@...aro.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/4] smp: Defer check for local execution in
 smp_call_function_many_cond()

On Fri, Jun 06, 2025 at 04:27:31PM -0400, Yury Norov wrote:
> From: "Yury Norov [NVIDIA]" <yury.norov@...il.com>
> 
> Defer check for local execution to the actual place where it is needed,
> and save some stack on a useless local variable.
> 
> Signed-off-by: Yury Norov [NVIDIA] <yury.norov@...il.com>
> ---
>  kernel/smp.c | 10 +++-------
>  1 file changed, 3 insertions(+), 7 deletions(-)
> 
> diff --git a/kernel/smp.c b/kernel/smp.c
> index 715190669e94..867f79689684 100644
> --- a/kernel/smp.c
> +++ b/kernel/smp.c
> @@ -779,7 +779,6 @@ static void smp_call_function_many_cond(const struct cpumask *mask,
>  	bool wait = scf_flags & SCF_WAIT;
>  	int nr_cpus = 0;
>  	bool run_remote = false;
> -	bool run_local = false;
>  
>  	lockdep_assert_preemption_disabled();
>  
> @@ -801,11 +800,6 @@ static void smp_call_function_many_cond(const struct cpumask *mask,
>  	 */
>  	WARN_ON_ONCE(!in_task());
>  
> -	/* Check if we need local execution. */
> -	if ((scf_flags & SCF_RUN_LOCAL) && cpumask_test_cpu(this_cpu, mask) &&
> -	    (!cond_func || cond_func(this_cpu, info)))
> -		run_local = true;
> -
>  	/* Check if we need remote execution, i.e., any CPU excluding this one. */
>  	if (cpumask_any_and_but(mask, cpu_online_mask, this_cpu) < nr_cpu_ids) {
>  		run_remote = true;
> @@ -853,7 +847,9 @@ static void smp_call_function_many_cond(const struct cpumask *mask,
>  			run_remote = false;
>  	}
>  
> -	if (run_local) {
> +	/* Check if we need local execution. */
> +	if ((scf_flags & SCF_RUN_LOCAL) & cpumask_test_cpu(this_cpu, mask) &&
> +	    (!cond_func || cond_func(this_cpu, info))) {

Dan Carpenter's robot pointed the bug here: it should be:

        (scf_flags & SCF_RUN_LOCAL) && cpumask_test_cpu(this_cpu, mask)

I'll resend it shortly.

Thanks, Dan!

>  		unsigned long flags;
>  
>  		local_irq_save(flags);
> -- 
> 2.43.0