[PATCHv2 2/3] rcu: Resort to cpu_dying_mask for affinity when offlining

[Pingfan Liu <kernelfans@gmail.com>]

During offlining, the concurrent rcutree_offline_cpu() can not be aware
of each other through ->qsmaskinitnext.  But cpu_dying_mask carries such
information at that point and can be utilized.

Besides, a trivial change which removes the redudant call to
rcu_boost_kthread_setaffinity() in rcutree_dead_cpu() since
rcutree_offline_cpu() can fully serve that purpose.

Signed-off-by: Pingfan Liu <kernelfans@gmail.com>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: David Woodhouse <dwmw@amazon.co.uk>
Cc: Frederic Weisbecker <frederic@kernel.org>
Cc: Neeraj Upadhyay <quic_neeraju@quicinc.com>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: "Jason A. Donenfeld" <Jason@zx2c4.com>
To: rcu@vger.kernel.org
---
 kernel/rcu/tree.c        | 2 --
 kernel/rcu/tree_plugin.h | 6 ++++++
 2 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 79aea7df4345..8a829b64f5b2 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2169,8 +2169,6 @@ int rcutree_dead_cpu(unsigned int cpu)
 		return 0;
 
 	WRITE_ONCE(rcu_state.n_online_cpus, rcu_state.n_online_cpus - 1);
-	/* Adjust any no-longer-needed kthreads. */
-	rcu_boost_kthread_setaffinity(rnp, -1);
 	// Stop-machine done, so allow nohz_full to disable tick.
 	tick_dep_clear(TICK_DEP_BIT_RCU);
 	return 0;
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index ef6d3ae239b9..e5afc63bd97f 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -1243,6 +1243,12 @@ static void rcu_boost_kthread_setaffinity(struct rcu_node *rnp, int outgoingcpu)
 		    cpu != outgoingcpu)
 			cpumask_set_cpu(cpu, cm);
 	cpumask_and(cm, cm, housekeeping_cpumask(HK_TYPE_RCU));
+	/*
+	 * For concurrent offlining, bit of qsmaskinitnext is not cleared yet.
+	 * So resort to cpu_dying_mask, whose changes has already been visible.
+	 */
+	if (outgoingcpu != -1)
+		cpumask_andnot(cm, cm, cpu_dying_mask);
 	if (cpumask_empty(cm))
 		cpumask_copy(cm, housekeeping_cpumask(HK_TYPE_RCU));
 	set_cpus_allowed_ptr(t, cm);
-- 
2.31.1