[PATCH 1/3] rcu: remove redundant cpu affinity setting during teardown

[Pingfan Liu <kernelfans@gmail.com>]

At present, during the cpu teardown, rcu_boost_kthread_setaffinity() is
called twice. Firstly by rcutree_offline_cpu(), then by
rcutree_dead_cpu() as the CPUHP_RCUTREE_PREP  cpuhp_step callback.

From the scheduler's perspective, a bit in cpu_online_mask means that the cpu
is visible to the scheduler. Furthermore, a bit in cpu_active_mask
means that the cpu is suitable as a migration destination.

Now turning back to the case in rcu offlining. sched_cpu_deactivate()
has disabled the dying cpu as the migration destination before
rcutree_offline_cpu().  Furthermore, if the boost kthread is on the dying
cpu, it will be migrated to another suitable online cpu by the scheduler.
So the affinity setting by rcutree_offline_cpu() is redundant and can be
eliminated.

Besides, this patch does an trival code rearrangement by unfolding
rcutree_affinity_setting() into rcutree_online_cpu(), considering that
the latter one is the only user of the former.

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 | 14 +-------------
 1 file changed, 1 insertion(+), 13 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 79aea7df4345..b90f6487fd45 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3978,16 +3978,6 @@ int rcutree_prepare_cpu(unsigned int cpu)
 	return 0;
 }
 
-/*
- * Update RCU priority boot kthread affinity for CPU-hotplug changes.
- */
-static void rcutree_affinity_setting(unsigned int cpu, int outgoing)
-{
-	struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
-
-	rcu_boost_kthread_setaffinity(rdp->mynode, outgoing);
-}
-
 /*
  * Near the end of the CPU-online process.  Pretty much all services
  * enabled, and the CPU is now very much alive.
@@ -4006,7 +3996,7 @@ int rcutree_online_cpu(unsigned int cpu)
 	if (rcu_scheduler_active == RCU_SCHEDULER_INACTIVE)
 		return 0; /* Too early in boot for scheduler work. */
 	sync_sched_exp_online_cleanup(cpu);
-	rcutree_affinity_setting(cpu, -1);
+	rcu_boost_kthread_setaffinity(rdp->mynode, -1);
 
 	// Stop-machine done, so allow nohz_full to disable tick.
 	tick_dep_clear(TICK_DEP_BIT_RCU);
@@ -4029,8 +4019,6 @@ int rcutree_offline_cpu(unsigned int cpu)
 	rnp->ffmask &= ~rdp->grpmask;
 	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 
-	rcutree_affinity_setting(cpu, cpu);
-
 	// nohz_full CPUs need the tick for stop-machine to work quickly
 	tick_dep_set(TICK_DEP_BIT_RCU);
 	return 0;
-- 
2.31.1