linux-kernel - [PATCH tip/core/rcu 13/21] rcu/nocb: Locally accelerate callbacks as long as offloading isn't complete

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Date:   Tue,  5 Jan 2021 20:48:45 -0800
From:   paulmck@...nel.org
To:     rcu@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, kernel-team@...com, mingo@...nel.org,
        jiangshanlai@...il.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,
        Frederic Weisbecker <frederic@...nel.org>,
        Neeraj Upadhyay <neeraju@...eaurora.org>,
        "Paul E . McKenney" <paulmck@...nel.org>
Subject: [PATCH tip/core/rcu 13/21] rcu/nocb: Locally accelerate callbacks as long as offloading isn't complete

From: Frederic Weisbecker <frederic@...nel.org>

The local callbacks processing checks if any callbacks need acceleration.
This commit carries out this checking under nocb lock protection in
the middle of toggle operations, during which time rcu_core() executes
concurrently with GP/CB kthreads.

Cc: Josh Triplett <josh@...htriplett.org>
Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: Lai Jiangshan <jiangshanlai@...il.com>
Cc: Joel Fernandes <joel@...lfernandes.org>
Cc: Neeraj Upadhyay <neeraju@...eaurora.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Inspired-by: Paul E. McKenney <paulmck@...nel.org>
Tested-by: Boqun Feng <boqun.feng@...il.com>
Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
---
 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 ec14c01..03810a5 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2699,7 +2699,6 @@ static __latent_entropy void rcu_core(void)
 	unsigned long flags;
 	struct rcu_data *rdp = raw_cpu_ptr(&rcu_data);
 	struct rcu_node *rnp = rdp->mynode;
-	const bool offloaded = rcu_segcblist_is_offloaded(&rdp->cblist);
 	const bool do_batch = !rcu_segcblist_completely_offloaded(&rdp->cblist);
 
 	if (cpu_is_offline(smp_processor_id()))
@@ -2720,11 +2719,11 @@ static __latent_entropy void rcu_core(void)
 
 	/* No grace period and unregistered callbacks? */
 	if (!rcu_gp_in_progress() &&
-	    rcu_segcblist_is_enabled(&rdp->cblist) && !offloaded) {
-		local_irq_save(flags);
+	    rcu_segcblist_is_enabled(&rdp->cblist) && do_batch) {
+		rcu_nocb_lock_irqsave(rdp, flags);
 		if (!rcu_segcblist_restempty(&rdp->cblist, RCU_NEXT_READY_TAIL))
 			rcu_accelerate_cbs_unlocked(rnp, rdp);
-		local_irq_restore(flags);
+		rcu_nocb_unlock_irqrestore(rdp, flags);
 	}
 
 	rcu_check_gp_start_stall(rnp, rdp, rcu_jiffies_till_stall_check());
-- 
2.9.5