On Fri, Mar 14, 2025 at 03:36:42PM +0100, Frederic Weisbecker wrote:
> A CPU within hotplug operations can make the RCU exp kworker lagging if:
>
> * The dying CPU is running after CPUHP_TEARDOWN_CPU but before
> rcutree_report_cpu_dead(). It is too late to send an IPI but RCU is
> still watching the CPU. Therefore the exp kworker can only wait for
> the target to reach rcutree_report_cpu_dead().
>
> * The booting CPU is running after rcutree_report_cpu_starting() but
> before set_cpu_online(). RCU is watching the CPU but it is too early
> to be able to send an IPI. Therefore the exp kworker can only wait
> until it observes the CPU as officially online.
>
> Such a lag is expected to be very short. However #VMEXIT and other
> hazards can stay on the way. Report long delays, 10 jiffies is
> considered a high threshold already.
>
> Reported-by: Paul E. McKenney <paul...@kernel.org>
> Signed-off-by: Frederic Weisbecker <frede...@kernel.org>
Same CONFIG_PROVE_RCU question, same conditional:
Reviewed-by: Paul E. McKenney <paul...@kernel.org>
> ---
> kernel/rcu/tree_exp.h | 10 ++++++++++
> 1 file changed, 10 insertions(+)
>
> diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
> index 6058a734090c..87a44423927d 100644
> --- a/kernel/rcu/tree_exp.h
> +++ b/kernel/rcu/tree_exp.h
> @@ -406,8 +406,18 @@ static void __sync_rcu_exp_select_node_cpus(struct
> rcu_exp_work *rewp)
> for_each_leaf_node_cpu_mask(rnp, cpu, mask_ofl_ipi) {
> struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
> unsigned long mask = rdp->grpmask;
> + int nr_retries = 0;
>
> retry_ipi:
> + /*
> + * In case of retrying, CPU either is lagging:
> + *
> + * - between CPUHP_TEARDOWN_CPU and rcutree_report_cpu_dead()
> + * or:
> + * - between rcutree_report_cpu_starting() and set_cpu_online()
> + */
> + WARN_ON_ONCE(nr_retries++ > 10);
> +
> if (rcu_watching_snap_stopped_since(rdp,
> rdp->exp_watching_snap)) {
> mask_ofl_test |= mask;
> continue;
> --
> 2.48.1
>
