linux-kernel - Re: [PATCH 02/11] smp: Enable preemption early in smp_call_function

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Message-Id: <98F9F4CE-210A-4270-A172-7B5D59DD64B3@linux.dev>
Date: Thu, 5 Feb 2026 11:55:23 +0800
From: Muchun Song <muchun.song@...ux.dev>
To: Chuyi Zhou <zhouchuyi@...edance.com>
Cc: tglx@...utronix.de,
 mingo@...hat.com,
 luto@...nel.org,
 peterz@...radead.org,
 paulmck@...nel.org,
 bp@...en8.de,
 dave.hansen@...ux.intel.com,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 02/11] smp: Enable preemption early in
 smp_call_function_single



> On Feb 3, 2026, at 19:23, Chuyi Zhou <zhouchuyi@...edance.com> wrote:
> 
> Now smp_call_function_single() disables preemption mainly for the following
> reasons:
> 
> - To protect the per-cpu csd_data from concurrent modification by other
> tasks on the current CPU in the !wait case. For the wait case,
> synchronization is not a concern as on-stack csd is used.
> 
> - To prevent the remote online CPU from being offlined. Specifically, we
> want to ensure that no new IPIs are queued after smpcfd_dying_cpu() has
> finished.
> 
> Disabling preemption for the entire execution is unnecessary, especially
> csd_lock_wait() part does not require preemption protection. This patch
> enables preemption before csd_lock_wait() to reduce the preemption-disabled
> critical section.
> 
> Signed-off-by: Chuyi Zhou <zhouchuyi@...edance.com>
> ---
> kernel/smp.c | 17 +++++++++++++++--
> 1 file changed, 15 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/smp.c b/kernel/smp.c
> index fc1f7a964616..0858553f3666 100644
> --- a/kernel/smp.c
> +++ b/kernel/smp.c
> @@ -685,11 +685,24 @@ int smp_call_function_single(int cpu, smp_call_func_t func, void *info,
> 
> 	err = generic_exec_single(cpu, csd);
> 
> + 	/*
> +	 * We may block in csd_lock_wait() for a significant amount of time (e.g., if the
> +	 * remote CPU has interrupts disabled). Disabling preemption throughout the entire
> +	 * smp_call_function_single() impacts the scheduling latency and is unnecessary.
> +	 *
> +	 * - Preemption must be disabled before sending the IPI to ensure no new IPIs are

            ^
That looks odd. Why use a dash ('-')?

> +	 * queued after smpcfd_dying_cpu() finishes.
> +	 *
> +	 * @csd is stack-allocated when @wait is true. No concurrent access except
> +	 * from the IPI completion path, so we can re-enable preemption early
> +	 * to reduce latency.
> +	 *

A blank line. Better to remove.

Reviewed-by: Muchun Song <muchun.song@...ux.dev>

Thanks.

> +	 */
> + 	put_cpu();
> +
> 	if (wait)
> 		csd_lock_wait(csd);
> 
> - 	put_cpu();
> -
> 	return err;
> }
> EXPORT_SYMBOL(smp_call_function_single);
> -- 
> 2.20.1