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From: "Joel Fernandes (Google)" <joel@joelfernandes.org>
To: linux-kernel@vger.kernel.org
Cc: "Joel Fernandes (Google)" <joel@joelfernandes.org>,
	Josh Triplett <josh@joshtriplett.org>,
	Lai Jiangshan <jiangshanlai@gmail.com>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Neeraj Upadhyay <neeraju@codeaurora.org>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	rcu@vger.kernel.org, Steven Rostedt <rostedt@goodmis.org>
Subject: [PATCH v2 3/3] rcu/tree: Make FQS complaining about offline CPU more aggressive
Date: Thu, 30 Jul 2020 20:40:12 -0400
Message-ID: <20200731004012.2324147-3-joel@joelfernandes.org> (raw)
In-Reply-To: <20200731004012.2324147-1-joel@joelfernandes.org>

Make FQS loop consider it an immediate failure if the case of an offline CPU
reporting QS is detected, instead of a full second.

This is because rcu_report_dead() already reports quiescent states and
updates ->qsmaskinitnext under node lock.

Light testing with TREE03 and hotplug shows no warnings.

Convert the warning as well to WARN_ON_ONCE() to reduce log spam.

Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
---
 kernel/rcu/tree.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index a621932cc385..39bdd744ba97 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1208,13 +1208,15 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
 		return 1;
 	}
 
-	/* If waiting too long on an offline CPU, complain. */
-	if (!(rdp->grpmask & rcu_rnp_online_cpus(rnp)) &&
-	    time_after(jiffies, rcu_state.gp_start + HZ)) {
+	/*
+	 * Complain if an offline CPU by RCU's books has not reported QS. Node
+	 * lock is held ensuring offlining does not race here.
+	 */
+	if (!(rdp->grpmask & rcu_rnp_online_cpus(rnp))) {
 		bool onl;
 		struct rcu_node *rnp1;
 
-		WARN_ON(1);  /* Offline CPUs are supposed to report QS! */
+		WARN_ON_ONCE(1);  /* Offline CPUs are supposed to report QS! */
 		pr_info("%s: grp: %d-%d level: %d ->gp_seq %ld ->completedqs %ld\n",
 			__func__, rnp->grplo, rnp->grphi, rnp->level,
 			(long)rnp->gp_seq, (long)rnp->completedqs);
-- 
2.28.0.163.g6104cc2f0b6-goog


  parent reply index

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-31  0:40 [PATCH v2 1/3] rcu/tree: Add a warning if CPU being onlined did not report QS already Joel Fernandes (Google)
2020-07-31  0:40 ` [PATCH v2 2/3] rcu/tree: Clarify comments about FQS loop reporting quiescent states Joel Fernandes (Google)
2020-07-31  0:40 ` Joel Fernandes (Google) [this message]
2020-07-31  1:36   ` [PATCH v2 3/3] rcu/tree: Make FQS complaining about offline CPU more aggressive Paul E. McKenney

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