Le Tue, Mar 18, 2025 at 10:22:33AM -0700, Paul E. McKenney a écrit :
> 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>
I don't have a strong opinion whether to keep this warning unconditional.
Perhaps this can depend on CONFIG_PROVE_RCU.
Thanks.
>
> > ---
> > 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
> >
