linux-kernel - [PATCH] rcu: Avoid unnecessary contention of rcu node lock

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Date:   Wed,  9 Nov 2016 17:57:13 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     paulmck@...ux.vnet.ibm.com, josh@...htriplett.org,
        rostedt@...dmis.org, mathieu.desnoyers@...icios.com,
        jiangshanlai@...il.com
Cc:     linux-kernel@...r.kernel.org
Subject: [PATCH] rcu: Avoid unnecessary contention of rcu node lock

It's unnecessary to try to print stacks of blocked tasks in the case
that ndetected == 0. Furthermore, calling rcu_print_detail_task_stall()
causes to acquire rnp locks as many times as the number of leaf nodes
plus one for root node. It's unnecessary at all in the case.

Signed-off-by: Byungchul Park <byungchul.park@....com>
---
 kernel/rcu/tree.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 287f468..ab2f743 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1374,6 +1374,9 @@ static void print_other_cpu_stall(struct rcu_state *rsp, unsigned long gpnum)
 	       (long)rsp->gpnum, (long)rsp->completed, totqlen);
 	if (ndetected) {
 		rcu_dump_cpu_stacks(rsp);
+
+		/* Complain about tasks blocking the grace period. */
+		rcu_print_detail_task_stall(rsp);
 	} else {
 		if (READ_ONCE(rsp->gpnum) != gpnum ||
 		    READ_ONCE(rsp->completed) == gpnum) {
@@ -1390,9 +1393,6 @@ static void print_other_cpu_stall(struct rcu_state *rsp, unsigned long gpnum)
 		}
 	}
 
-	/* Complain about tasks blocking the grace period. */
-	rcu_print_detail_task_stall(rsp);
-
 	rcu_check_gp_kthread_starvation(rsp);
 
 	panic_on_rcu_stall();
-- 
1.9.1