linux-kernel - [PATCH v2 tip/core/rcu 01/11] rcu: Short-circuit synchronize_sched

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Date:	Wed,  9 Dec 2015 15:02:58 -0800
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,
	dvhart@...ux.intel.com, fweisbec@...il.com, oleg@...hat.com,
	bobby.prani@...il.com,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH v2 tip/core/rcu 01/11] rcu: Short-circuit synchronize_sched_expedited() if only one CPU

If there is only one CPU, then invoking synchronize_sched_expedited()
is by definition a grace period.  This commit checks for this condition
and does a short-circuit return in that case.

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

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 81aa1cdc6bc9..bd2605c144cc 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3822,6 +3822,10 @@ void synchronize_sched_expedited(void)
 	struct rcu_node *rnp;
 	struct rcu_state *rsp = &rcu_sched_state;
 
+	/* If only one CPU, this is automatically a grace period. */
+	if (rcu_blocking_is_gp())
+		return;
+
 	/* Take a snapshot of the sequence number.  */
 	s = rcu_exp_gp_seq_snap(rsp);
 
-- 
2.5.2

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