[PATCH v6 4/4] percpu-refcount: Use call_rcu_flush() for atomic switch

Joel Fernandes (Google)
2022-09-22T22:01:55Z

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@joelfernandes.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