Re: [PATCH rcu 3/3] rcu: Allow expedited RCU grace periods on incoming CPUs

[Frederic Weisbecker]

On Fri, Feb 04, 2022 at 02:55:07PM -0800, Paul E. McKenney wrote:
> Although it is usually safe to invoke synchronize_rcu_expedited() from a
> preemption-enabled CPU-hotplug notifier, if it is invoked from a notifier
> between CPUHP_AP_RCUTREE_ONLINE and CPUHP_AP_ACTIVE, its attempts to
> invoke a workqueue handler will hang due to RCU waiting on a CPU that
> the scheduler is not paying attention to.  This commit therefore expands
> use of the existing workqueue-independent synchronize_rcu_expedited()
> from early boot to also include CPUs that are being hotplugged.
> 
> Link: https://lore.kernel.org/lkml/7359f994-8aaf-3cea-f5cf-c0d3929689d6@xxxxxxxxxxx/
> Reported-by: Mukesh Ojha <quic_mojha@xxxxxxxxxxx>
> Cc: Tejun Heo <tj@xxxxxxxxxx>
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>

I'm surprised by this scheduler behaviour.

Since sched_cpu_activate() hasn't been called yet,
rq->balance_callback = balance_push_callback. As a result, balance_push() should
be called at the end of schedule() when the workqueue is picked as the next task.
Then eventually the workqueue should be immediately preempted by the stop task to
be migrated elsewhere.

So I must be missing something. For the fun, I booted the following and it
didn't produce any issue:

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 80faf2273ce9..b1e74a508881 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -4234,6 +4234,8 @@ int rcutree_online_cpu(unsigned int cpu)
 
 	// Stop-machine done, so allow nohz_full to disable tick.
 	tick_dep_clear(TICK_DEP_BIT_RCU);
+	if (cpu != 0)
+		synchronize_rcu_expedited();
 	return 0;
 }