Re: [PATCH 5/5] rcu/exp: Warn on CPU lagging for too long within hotplug IPI's blindspot

[Joel Fernandes <joelagnelf@nvidia.com>]

> On Apr 29, 2025, at 9:44 AM, Frederic Weisbecker <frederic@kernel.org> wrote:

Hi Frederic,
These all look good to me. Do you wish for
these to go into the upcoming merge window or
can I push them to a for-Neeraj branch as he
is doing the merge window after the next?

Thanks,

 - Joel


> 
> 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 <paulmck@kernel.org>
> Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
> Signed-off-by: Frederic Weisbecker <frederic@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
>