linux-kernel - [PATCH 05/19] rcu/nocb: Don't deoffload an offline CPU with pending work

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Date:   Fri, 13 Nov 2020 13:13:20 +0100
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>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Neeraj Upadhyay <neeraju@...eaurora.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Josh Triplett <josh@...htriplett.org>, rcu@...r.kernel.org
Subject: [PATCH 05/19] rcu/nocb: Don't deoffload an offline CPU with pending work

Offlining offloaded CPUs don't migrate their callbacks just like
non-offloaded CPUs do. It's up to their CB/GP kthread to handle what
remains.

Therefore we can't afford to de-offload an offline CPU that still has
pending work to do, or the callbacks would be ignored.

NOTE: The long term solution will be to wait for all pending callbacks
to be processed before completing a CPU down operation.

Suggested-by: Paul E. McKenney <paulmck@...nel.org>
Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
Cc: Paul E. McKenney <paulmck@...nel.org>
Cc: Josh Triplett <josh@...htriplett.org>
Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: Lai Jiangshan <jiangshanlai@...il.com>
Cc: Joel Fernandes <joel@...lfernandes.org>
Cc: Neeraj Upadhyay <neeraju@...eaurora.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Boqun Feng <boqun.feng@...il.com>
---
 kernel/rcu/tree_plugin.h | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 08eb035da9e7..5075bf219b23 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -2227,6 +2227,15 @@ static int __rcu_nocb_rdp_deoffload(struct rcu_data *rdp)
 	printk("De-offloading %d\n", rdp->cpu);
 
 	rcu_nocb_lock_irqsave(rdp, flags);
+	/*
+	 * If there are still pending work offloaded, the offline
+	 * CPU won't help much handling them.
+	 */
+	if (cpu_is_offline(rdp->cpu) && !rcu_segcblist_empty(&rdp->cblist)) {
+		rcu_nocb_unlock_irqrestore(rdp, flags);
+		return -EBUSY;
+	}
+
 	rcu_segcblist_offload(cblist, false);
 
 	if (rdp->nocb_cb_sleep) {
-- 
2.25.1