Re: [PATCH 02/11] smp: Enable preemption early in smp_call_function_single

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Feb 03, 2026 at 07:23:52PM +0800, Chuyi Zhou 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@bytedance.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
> +	 * 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.
> +	 *
> +	 */

Those lines are too long for no reason, please reflow at 78 or so.