Re: [PATCH] rcu-tasks: Warn on too-short RCU Tasks Rude grace period

[Joel Fernandes <joelagnelf@nvidia.com>]

> On Oct 15, 2026, at 5:46 AM, Zqiang <qiang.zhang@linux.dev> wrote:
> 
> The RCU Tasks Rude grace period is implemented by invoking the
> schedule_on_each_cpu() with an empty work function, which forces
> a context switch on every online CPU and guarantees that all
> rude-tasks read-side critical sections have completed. However,
> schedule_on_each_cpu() can fail (via alloc_percpu() returning
> -ENOMEM) and returns immediately, in which case those context
> switches never happen, too-short grace period happen.
> 
> This commit therefore wraps the schedule_on_each_cpu() call in
> WARN_ONCE() so that this failure is reported as a "Too-short
> grace period" warning.
> 
> Signed-off-by: Zqiang <qiang.zhang@linux.dev>
> ---
> kernel/rcu/tasks.h | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
> index fcac7361ec51..91c25717402e 100644
> --- a/kernel/rcu/tasks.h
> +++ b/kernel/rcu/tasks.h
> @@ -1361,7 +1361,8 @@ static void rcu_tasks_be_rude(struct work_struct *work)
> static void rcu_tasks_rude_wait_gp(struct rcu_tasks *rtp)
> {
>    rtp->n_ipis += cpumask_weight(cpu_online_mask);
> -    schedule_on_each_cpu(rcu_tasks_be_rude);
> +    WARN_ONCE(schedule_on_each_cpu(rcu_tasks_be_rude),
> +          "%s: Too-short grace period, OOM is now expected behavior!!!\n", rtp->name);

Is that the best error message? OOM is expected when grace periods do not conclude.

Thanks.

> }
> 
> static void call_rcu_tasks_rude(struct rcu_head *rhp, rcu_callback_t func);
> --
> 2.17.1
>