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From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     rcu@vger.kernel.org
Cc:     linux-kernel@vger.kernel.org, kernel-team@fb.com,
        rostedt@goodmis.org,
        "Joel Fernandes (Google)" <joel@joelfernandes.org>,
        "Paul E . McKenney" <paulmck@kernel.org>
Subject: [PATCH rcu 08/14] percpu-refcount: Use call_rcu_flush() for atomic switch
Date:   Wed, 19 Oct 2022 15:51:38 -0700
Message-Id: <20221019225144.2500095-8-paulmck@kernel.org>
In-Reply-To: <20221019225138.GA2499943@paulmck-ThinkPad-P17-Gen-1>
References: <20221019225138.GA2499943@paulmck-ThinkPad-P17-Gen-1>
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From: "Joel Fernandes (Google)" <joel@joelfernandes.org>

call_rcu() changes to save power will slow down the percpu refcounter's
"per-CPU to atomic switch" path. The primitive uses RCU when switching to
atomic mode. The enqueued async callback wakes up waiters waiting in the
percpu_ref_switch_waitq. Due to this, per-CPU refcount users will slow down,
such as blk_pre_runtime_suspend().

Use the call_rcu_flush() API instead which reverts to the old behavior.

Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
 lib/percpu-refcount.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/lib/percpu-refcount.c b/lib/percpu-refcount.c
index e5c5315da2741..65c58a029297d 100644
--- a/lib/percpu-refcount.c
+++ b/lib/percpu-refcount.c
@@ -230,7 +230,8 @@ static void __percpu_ref_switch_to_atomic(struct percpu_ref *ref,
 		percpu_ref_noop_confirm_switch;
 
 	percpu_ref_get(ref);	/* put after confirmation */
-	call_rcu(&ref->data->rcu, percpu_ref_switch_to_atomic_rcu);
+	call_rcu_flush(&ref->data->rcu,
+		       percpu_ref_switch_to_atomic_rcu);
 }
 
 static void __percpu_ref_switch_to_percpu(struct percpu_ref *ref)
-- 
2.31.1.189.g2e36527f23