linux-kernel - [PATCH tip/core/rcu 08/14] rcu: Trace end of grace period before end of grace period

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Date:   Sun, 11 Nov 2018 11:28:58 -0800
From:   "Paul E. McKenney" <paulmck@...ux.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, joel@...lfernandes.org,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>
Subject: [PATCH tip/core/rcu 08/14] rcu: Trace end of grace period before end of grace period

Currently, rcu_gp_cleanup() traces the end of the old grace period after
the old grace period has officially ended.  This might make intuitive
sense, but it also makes for confusing event-trace output because the
"end" trace displays not the old but instead the new grace-period number.
This commit therefore traces the end of an old grace period just before
that grace period officially ends.

Reported-by: Aravinda Prasad <aravinda@...ux.vnet.ibm.com>
Signed-off-by: Paul E. McKenney <paulmck@...ux.ibm.com>
---
 kernel/rcu/tree.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 429a46fb087a..61bae41b1afe 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2035,9 +2035,9 @@ static void rcu_gp_cleanup(void)
 	rnp = rcu_get_root();
 	raw_spin_lock_irq_rcu_node(rnp); /* GP before ->gp_seq update. */
 
-	/* Declare grace period done. */
-	rcu_seq_end(&rcu_state.gp_seq);
+	/* Declare grace period done, trace first to use old GP number. */
 	trace_rcu_grace_period(rcu_state.name, rcu_state.gp_seq, TPS("end"));
+	rcu_seq_end(&rcu_state.gp_seq);
 	rcu_state.gp_state = RCU_GP_IDLE;
 	/* Check for GP requests since above loop. */
 	rdp = this_cpu_ptr(&rcu_data);
-- 
2.17.1