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

[Uladzislau Rezki <urezki@gmail.com>]

On Wed, May 20, 2026 at 02:04:01PM +0200, Frederic Weisbecker wrote:
> Le Tue, May 19, 2026 at 09:01:28PM +0200, Arnd Bergmann a écrit :
> > From: Arnd Bergmann <arnd@arndb.de>
> > 
> > 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@arndb.de>
> > ---
> >  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@kernel.org>
> 
Took the patch and applied the tag from both!

--
Uladzislau Rezki