From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Date: Fri, 8 Nov 2019 18:35:53 +0100
Subject: percpu-refcount: Use normal instead of RCU-sched"
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Patch-mainline: v5.5-rc1
Git-commit: 9e8d42a0f7eb9056f8bdb241b91738b5a2923f4c
References: bsc#1162702

This is a revert of commit
   a4244454df129 ("percpu-refcount: use RCU-sched insted of normal RCU")

which claims the only reason for using RCU-sched is
   "rcu_read_[un]lock() … are slightly more expensive than preempt_disable/enable()"

and
    "As the RCU critical sections are extremely short, using sched-RCU
    shouldn't have any latency implications."

The problem with using RCU-sched here is that it disables preemption and
the release callback (called from percpu_ref_put_many()) must not
acquire any sleeping locks like spinlock_t. This breaks PREEMPT_RT
because some of the users acquire spinlock_t locks in their callbacks.

Using rcu_read_lock() on PREEMPTION=n kernels is not any different
compared to rcu_read_lock_sched(). On PREEMPTION=y kernels there are
already performance issues due to additional preemption points.
Looking at the code, the rcu_read_lock() is just an increment and unlock
is almost just a decrement unless there is something special to do. Both
are functions while disabling preemption is inlined.
Doing a small benchmark, the minimal amount of time required was mostly
the same. The average time required was higher due to the higher MAX
value (which could be preemption). With DEBUG_PREEMPT=y it is
rcu_read_lock_sched() that takes a little longer due to the additional
debug code.

Convert back to normal RCU.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Dennis Zhou <dennis@kernel.org>
Acked-by: Daniel Wagner <dwagner@suse.de>
---
 include/linux/percpu-refcount.h |   16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

--- a/include/linux/percpu-refcount.h
+++ b/include/linux/percpu-refcount.h
@@ -186,14 +186,14 @@ static inline void percpu_ref_get_many(s
 {
 	unsigned long __percpu *percpu_count;
 
-	rcu_read_lock_sched();
+	rcu_read_lock();
 
 	if (__ref_is_percpu(ref, &percpu_count))
 		this_cpu_add(*percpu_count, nr);
 	else
 		atomic_long_add(nr, &ref->count);
 
-	rcu_read_unlock_sched();
+	rcu_read_unlock();
 }
 
 /**
@@ -223,7 +223,7 @@ static inline bool percpu_ref_tryget(str
 	unsigned long __percpu *percpu_count;
 	bool ret;
 
-	rcu_read_lock_sched();
+	rcu_read_lock();
 
 	if (__ref_is_percpu(ref, &percpu_count)) {
 		this_cpu_inc(*percpu_count);
@@ -232,7 +232,7 @@ static inline bool percpu_ref_tryget(str
 		ret = atomic_long_inc_not_zero(&ref->count);
 	}
 
-	rcu_read_unlock_sched();
+	rcu_read_unlock();
 
 	return ret;
 }
@@ -257,7 +257,7 @@ static inline bool percpu_ref_tryget_liv
 	unsigned long __percpu *percpu_count;
 	bool ret = false;
 
-	rcu_read_lock_sched();
+	rcu_read_lock();
 
 	if (__ref_is_percpu(ref, &percpu_count)) {
 		this_cpu_inc(*percpu_count);
@@ -266,7 +266,7 @@ static inline bool percpu_ref_tryget_liv
 		ret = atomic_long_inc_not_zero(&ref->count);
 	}
 
-	rcu_read_unlock_sched();
+	rcu_read_unlock();
 
 	return ret;
 }
@@ -285,14 +285,14 @@ static inline void percpu_ref_put_many(s
 {
 	unsigned long __percpu *percpu_count;
 
-	rcu_read_lock_sched();
+	rcu_read_lock();
 
 	if (__ref_is_percpu(ref, &percpu_count))
 		this_cpu_sub(*percpu_count, nr);
 	else if (unlikely(atomic_long_sub_and_test(nr, &ref->count)))
 		ref->release(ref);
 
-	rcu_read_unlock_sched();
+	rcu_read_unlock();
 }
 
 /**