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

[Kunwu Chan]

On 5/20/26 03:01, Arnd Bergmann wrote:
> 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)
>  {
>  	bool bypass = false;
>  	int __maybe_unused cpu = my_rdp->cpu;

Reviewed-by: Kunwu Chan <chentao@xxxxxxxxxx>


-- 
Thanx, Kunwu