* Re: [PATCH] rcu/nocb: reduce stack usage in nocb_gp_wait()
2026-05-19 19:01 [PATCH] rcu/nocb: reduce stack usage in nocb_gp_wait() Arnd Bergmann
@ 2026-05-19 21:52 ` Paul E. McKenney
2026-05-20 12:04 ` Frederic Weisbecker
2026-05-27 12:07 ` Kunwu Chan
2 siblings, 0 replies; 5+ messages in thread
From: Paul E. McKenney @ 2026-05-19 21:52 UTC (permalink / raw)
To: Arnd Bergmann
Cc: Frederic Weisbecker, Neeraj Upadhyay, Joel Fernandes,
Josh Triplett, Boqun Feng, Uladzislau Rezki, Arnd Bergmann,
Steven Rostedt, Mathieu Desnoyers, Lai Jiangshan, Zqiang,
Ingo Molnar, Tze-nan Wu, rcu, linux-kernel
On Tue, May 19, 2026 at 09:01:28PM +0200, Arnd Bergmann wrote:
> 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>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
> ---
> 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;
> --
> 2.39.5
>
^ permalink raw reply [flat|nested] 5+ messages in thread* Re: [PATCH] rcu/nocb: reduce stack usage in nocb_gp_wait()
2026-05-19 19:01 [PATCH] rcu/nocb: reduce stack usage in nocb_gp_wait() Arnd Bergmann
2026-05-19 21:52 ` Paul E. McKenney
@ 2026-05-20 12:04 ` Frederic Weisbecker
2026-05-20 14:02 ` Uladzislau Rezki
2026-05-27 12:07 ` Kunwu Chan
2 siblings, 1 reply; 5+ messages in thread
From: Frederic Weisbecker @ 2026-05-20 12:04 UTC (permalink / raw)
To: Arnd Bergmann
Cc: Paul E. McKenney, Neeraj Upadhyay, Joel Fernandes, Josh Triplett,
Boqun Feng, Uladzislau Rezki, Arnd Bergmann, Steven Rostedt,
Mathieu Desnoyers, Lai Jiangshan, Zqiang, Ingo Molnar, Tze-nan Wu,
rcu, linux-kernel
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>
> {
> bool bypass = false;
> int __maybe_unused cpu = my_rdp->cpu;
> --
> 2.39.5
>
--
Frederic Weisbecker
SUSE Labs
^ permalink raw reply [flat|nested] 5+ messages in thread* Re: [PATCH] rcu/nocb: reduce stack usage in nocb_gp_wait()
2026-05-20 12:04 ` Frederic Weisbecker
@ 2026-05-20 14:02 ` Uladzislau Rezki
0 siblings, 0 replies; 5+ messages in thread
From: Uladzislau Rezki @ 2026-05-20 14:02 UTC (permalink / raw)
To: Frederic Weisbecker, Arnd Bergmann, Paul E. McKenney
Cc: Arnd Bergmann, Paul E. McKenney, Neeraj Upadhyay, Joel Fernandes,
Josh Triplett, Boqun Feng, Uladzislau Rezki, Arnd Bergmann,
Steven Rostedt, Mathieu Desnoyers, Lai Jiangshan, Zqiang,
Ingo Molnar, Tze-nan Wu, rcu, linux-kernel
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
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [PATCH] rcu/nocb: reduce stack usage in nocb_gp_wait()
2026-05-19 19:01 [PATCH] rcu/nocb: reduce stack usage in nocb_gp_wait() Arnd Bergmann
2026-05-19 21:52 ` Paul E. McKenney
2026-05-20 12:04 ` Frederic Weisbecker
@ 2026-05-27 12:07 ` Kunwu Chan
2 siblings, 0 replies; 5+ messages in thread
From: Kunwu Chan @ 2026-05-27 12:07 UTC (permalink / raw)
To: Arnd Bergmann, Paul E. McKenney, Frederic Weisbecker,
Neeraj Upadhyay, Joel Fernandes, Josh Triplett, Boqun Feng,
Uladzislau Rezki
Cc: Arnd Bergmann, Steven Rostedt, Mathieu Desnoyers, Lai Jiangshan,
Zqiang, Ingo Molnar, Tze-nan Wu, rcu, linux-kernel
On 5/20/26 03:01, Arnd Bergmann wrote:
> 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)
> {
> bool bypass = false;
> int __maybe_unused cpu = my_rdp->cpu;
Reviewed-by: Kunwu Chan <chentao@kylinos.cn>
--
Thanx, Kunwu
^ permalink raw reply [flat|nested] 5+ messages in thread