linux-kernel - [PATCH tip/core/rcu 05/12] rcu: Check for NOCB CPUs and empty lists earlier in CB migration

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Message-Id: <1500933116-11642-5-git-send-email-paulmck@linux.vnet.ibm.com>
Date:   Mon, 24 Jul 2017 14:51:49 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     linux-kernel@...r.kernel.org
Cc:     mingo@...nel.org, jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
        rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
        fweisbec@...il.com, oleg@...hat.com,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH tip/core/rcu 05/12] rcu: Check for NOCB CPUs and empty lists earlier in CB migration

The current CPU-hotplug RCU-callback-migration code checks
for the source (newly offlined) CPU being a NOCBs CPU down in
rcu_send_cbs_to_orphanage().  This commit simplifies callback migration a
bit by moving this check up to rcu_migrate_callbacks().  This commit also
adds a check for the source CPU having no callbacks, which eases analysis
of the rcu_send_cbs_to_orphanage() and rcu_adopt_orphan_cbs() functions.

Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
---
 kernel/rcu/tree.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index f5acf34247fb..aeea697d6f9f 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3862,10 +3862,6 @@ rcu_send_cbs_to_orphanage(int cpu, struct rcu_state *rsp,
 {
 	lockdep_assert_held(&rsp->orphan_lock);
 
-	/* No-CBs CPUs do not have orphanable callbacks. */
-	if (!IS_ENABLED(CONFIG_HOTPLUG_CPU) || rcu_is_nocb_cpu(rdp->cpu))
-		return;
-
 	/*
 	 * Orphan the callbacks.  First adjust the counts.  This is safe
 	 * because _rcu_barrier() excludes CPU-hotplug operations, so it
@@ -3935,6 +3931,9 @@ static void rcu_migrate_callbacks(int cpu, struct rcu_state *rsp)
 	struct rcu_data *rdp = per_cpu_ptr(rsp->rda, cpu);
 	struct rcu_node *rnp = rdp->mynode;  /* Outgoing CPU's rdp & rnp. */
 
+	if (rcu_is_nocb_cpu(cpu) || rcu_segcblist_empty(&rdp->cblist))
+		return;  /* No callbacks to migrate. */
+
 	raw_spin_lock_irqsave(&rsp->orphan_lock, flags);
 	rcu_send_cbs_to_orphanage(cpu, rsp, rnp, rdp);
 	rcu_adopt_orphan_cbs(rsp, flags);
-- 
2.5.2