Re: [PATCH] rcu/nocb: reduce stack usage in nocb_gp_wait()

[Frederic Weisbecker]

Le Tue, May 19, 2026 at 09:01:28PM +0200, Arnd Bergmann a écrit :
> From: Arnd Bergmann <arnd@xxxxxxxx>
> 
> When CONFIG_UBSAN_ALIGNMENT is enabled, the stack usage of nocb_gp_wait()
> grows above typical warning limits:
> 
> In file included from kernel/rcu/tree.c:4930:
> kernel/rcu/tree_nocb.h: In function 'rcu_nocb_gp_kthread':
> kernel/rcu/tree_nocb.h:866:1: error: the frame size of 1968 bytes is larger than 1280 bytes [-Werror=frame-larger-than=]
> 
> Apparently, the problem is passing rcu_data from a 'void *' pointer,
> which gcc assumes may be misaligned. When the function is not inlined
> into rcu_nocb_gp_kthread(), that is no longer visible to gcc.
> 
> Add a 'noinline_for_stack' annotation that leads to skipping a lot of
> the alignment sanitizer checks and keeps the stack usage 60% lower here.
> 
> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
> ---
>  kernel/rcu/tree_nocb.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> index 1047b30cd46b..373b877cf171 100644
> --- a/kernel/rcu/tree_nocb.h
> +++ b/kernel/rcu/tree_nocb.h
> @@ -655,7 +655,7 @@ static void nocb_gp_sleep(struct rcu_data *my_rdp, int cpu)
>   * No-CBs GP kthreads come here to wait for additional callbacks to show up
>   * or for grace periods to end.
>   */
> -static void nocb_gp_wait(struct rcu_data *my_rdp)
> +static noinline_for_stack void nocb_gp_wait(struct rcu_data *my_rdp)

This probably deserve a comment though.

Other than that:

Reviewed-by: Frederic Weisbecker <frederic@xxxxxxxxxx>


>  {
>  	bool bypass = false;
>  	int __maybe_unused cpu = my_rdp->cpu;
> -- 
> 2.39.5
> 

-- 
Frederic Weisbecker
SUSE Labs