linux-kernel - Re: [PATCH 04/11] smp: Use on-stack cpumask in smp_call_function_many

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Message-ID: <20260205094422.GG232055@noisy.programming.kicks-ass.net>
Date: Thu, 5 Feb 2026 10:44:22 +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 04/11] smp: Use on-stack cpumask in
 smp_call_function_many_cond

On Tue, Feb 03, 2026 at 07:23:54PM +0800, Chuyi Zhou wrote:
> This patch use on-stack cpumask to replace percpu cfd cpumask in
> smp_call_function_many_cond(). alloc_cpumask_var() may fail when
> CONFIG_CPUMASK_OFFSTACK is enabled. In such extreme case, fall back to
> cfd->cpumask. This is a preparation for the next patch.
> 
> Signed-off-by: Chuyi Zhou <zhouchuyi@...edance.com>
> ---
>  kernel/smp.c | 22 +++++++++++++++++-----
>  1 file changed, 17 insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/smp.c b/kernel/smp.c
> index f572716c3c7d..35948afced2e 100644
> --- a/kernel/smp.c
> +++ b/kernel/smp.c
> @@ -805,11 +805,17 @@ static void smp_call_function_many_cond(const struct cpumask *mask,
>  	int cpu, last_cpu, this_cpu = smp_processor_id();
>  	struct call_function_data *cfd;
>  	bool wait = scf_flags & SCF_WAIT;
> +	bool preemptible_wait = true;
> +	cpumask_var_t cpumask_stack;
> +	struct cpumask *cpumask;
>  	int nr_cpus = 0;
>  	bool run_remote = false;
>  
>  	lockdep_assert_preemption_disabled();
>  
> +	if (!alloc_cpumask_var(&cpumask_stack, GFP_ATOMIC))
> +		preemptible_wait = false;

IIRC this breaks RT, must not allocate with preemption disabled.