linux-kernel - [PATCH 2/3] rcu/nocb: Remove NOCB deferred wakeup from rcutree_dead

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Date:   Wed, 19 May 2021 02:09:29 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     "Paul E . McKenney" <paulmck@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Frederic Weisbecker <frederic@...nel.org>, rcu@...r.kernel.org
Subject: [PATCH 2/3] rcu/nocb: Remove NOCB deferred wakeup from rcutree_dead_cpu()

At CPU offline time, we make sure to flush 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@...nel.org>
---
 kernel/rcu/tree.c | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 28f1093027b9..a6b448e6e059 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2469,9 +2469,6 @@ 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;
-- 
2.25.1