From 0991e0ca5870ae7a8edd43476bb1019f235d0e3d Mon Sep 17 00:00:00 2001
From: Frederic Weisbecker <frederic@kernel.org>
Date: Wed, 19 May 2021 02:09:29 +0200
Subject: [PATCH] rcu/nocb: Remove NOCB deferred wakeup from rcutree_dead_cpu()

References: bnc#1189998 (PREEMPT_RT prerequisite backports)
Patch-mainline: v5.15-rc1
Git-commit: cba712beebf32b27fea71241aa3cdd2ab0fc31a3

At CPU offline time, we must handle any pending wakeup for the nocb_gp
kthread linked to the outgoing CPU.

Now we are making sure of that twice:

1) From rcu_report_dead() when the outgoing CPU makes the very last
   local cleanups by itself before switching offline.

2) From rcutree_dead_cpu(). Here the offlining CPU has gone and is truly
   now offline. Another CPU takes care of post-portem cleaning up and
   check if the offline CPU had pending wakeup.

Both ways are fine but we have to choose one or the other because we
don't need to repeat that action. Simply benefit from cache locality
and keep only the first solution.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Mel Gorman <mgorman@suse.de>
---
 kernel/rcu/tree.c | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 66ea9f10fa5e..2709848f5c1f 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2432,9 +2432,6 @@ int rcutree_dead_cpu(unsigned int cpu)
 	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);
-	/* Do any needed no-CB deferred wakeups from this CPU. */
-	do_nocb_deferred_wakeup(per_cpu_ptr(&rcu_data, cpu));
-
 	// Stop-machine done, so allow nohz_full to disable tick.
 	tick_dep_clear(TICK_DEP_BIT_RCU);
 	return 0;