On Wed, Mar 19, 2025 at 10:42:38AM +0100, Frederic Weisbecker wrote:
> 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.
You are the expert on that question, so your choice. On this one,
I am but asking the questions. ;-)
Thanx, Paul
> 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
> > >
