linux-kernel - [PATCH v6 4/4] percpu-refcount: Use call_rcu

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Message-Id: <20220922220104.2446868-5-joel@joelfernandes.org>
Date:   Thu, 22 Sep 2022 22:01:04 +0000
From:   "Joel Fernandes (Google)" <joel@...lfernandes.org>
To:     rcu@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, rushikesh.s.kadam@...el.com,
        urezki@...il.com, neeraj.iitr10@...il.com, frederic@...nel.org,
        paulmck@...nel.org, rostedt@...dmis.org,
        "Joel Fernandes (Google)" <joel@...lfernandes.org>
Subject: [PATCH v6 4/4] percpu-refcount: Use call_rcu_flush() for atomic switch

call_rcu() changes to save power will slow down percpu refcounter
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. This will slow down the per-CPU refcount users
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@...lfernandes.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 e5c5315da274..65c58a029297 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)
-- 
2.37.3.998.g577e59143f-goog