percpu_ref: reduce memory footprint of percpu_ref in fast path

'struct percpu_ref' is often embedded into one user structure, and the
instance is usually referenced in fast path, however actually only
'percpu_count_ptr' is needed in fast path.

So move other fields into one new structure of 'percpu_ref_data', and
allocate it dynamically via kzalloc(), then memory footprint of
'percpu_ref' in fast path is reduced a lot and becomes suitable to put
into hot cacheline of user structure.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Tested-by: Veronika Kabatova <vkabatov@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Acked-by: Tejun Heo <tj@kernel.org>
Cc: Sagi Grimberg <sagi@grimberg.me>
Cc: Tejun Heo <tj@kernel.org>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 24 insertions, 28 deletions

diff --git a/include/linux/percpu-refcount.h b/include/linux/percpu-refcount.h
index 87d8a38bdea1..16c35a728b4c 100644
--- a/include/linux/percpu-refcount.h
+++ b/include/linux/percpu-refcount.h
@@ -92,18 +92,30 @@ enum {
 	PERCPU_REF_ALLOW_REINIT	= 1 << 2,
 };
 
-struct percpu_ref {
+struct percpu_ref_data {
 	atomic_long_t		count;
-	/*
-	 * The low bit of the pointer indicates whether the ref is in percpu
-	 * mode; if set, then get/put will manipulate the atomic_t.
-	 */
-	unsigned long		percpu_count_ptr;
 	percpu_ref_func_t	*release;
 	percpu_ref_func_t	*confirm_switch;
 	bool			force_atomic:1;
 	bool			allow_reinit:1;
 	struct rcu_head		rcu;
+	struct percpu_ref	*ref;
+};
+
+struct percpu_ref {
+	/*
+	 * The low bit of the pointer indicates whether the ref is in percpu
+	 * mode; if set, then get/put will manipulate the atomic_t.
+	 */
+	unsigned long		percpu_count_ptr;
+
+	/*
+	 * 'percpu_ref' is often embedded into user structure, and only
+	 * 'percpu_count_ptr' is required in fast path, move other fields
+	 * into 'percpu_ref_data', so we can reduce memory footprint in
+	 * fast path.
+	 */
+	struct percpu_ref_data  *data;
 };
 
 int __must_check percpu_ref_init(struct percpu_ref *ref,
@@ -118,6 +130,7 @@ void percpu_ref_kill_and_confirm(struct percpu_ref *ref,
 				 percpu_ref_func_t *confirm_kill);
 void percpu_ref_resurrect(struct percpu_ref *ref);
 void percpu_ref_reinit(struct percpu_ref *ref);
+bool percpu_ref_is_zero(struct percpu_ref *ref);
 
 /**
  * percpu_ref_kill - drop the initial ref
@@ -191,7 +204,7 @@ static inline void percpu_ref_get_many(struct percpu_ref *ref, unsigned long nr)
 	if (__ref_is_percpu(ref, &percpu_count))
 		this_cpu_add(*percpu_count, nr);
 	else
-		atomic_long_add(nr, &ref->count);
+		atomic_long_add(nr, &ref->data->count);
 
 	rcu_read_unlock();
 }
@@ -231,7 +244,7 @@ static inline bool percpu_ref_tryget_many(struct percpu_ref *ref,
 		this_cpu_add(*percpu_count, nr);
 		ret = true;
 	} else {
-		ret = atomic_long_add_unless(&ref->count, nr, 0);
+		ret = atomic_long_add_unless(&ref->data->count, nr, 0);
 	}
 
 	rcu_read_unlock();
@@ -279,7 +292,7 @@ static inline bool percpu_ref_tryget_live(struct percpu_ref *ref)
 		this_cpu_inc(*percpu_count);
 		ret = true;
 	} else if (!(ref->percpu_count_ptr & __PERCPU_REF_DEAD)) {
-		ret = atomic_long_inc_not_zero(&ref->count);
+		ret = atomic_long_inc_not_zero(&ref->data->count);
 	}
 
 	rcu_read_unlock();
@@ -305,8 +318,8 @@ static inline void percpu_ref_put_many(struct percpu_ref *ref, unsigned long nr)
 
 	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);
+	else if (unlikely(atomic_long_sub_and_test(nr, &ref->data->count)))
+		ref->data->release(ref);
 
 	rcu_read_unlock();
 }
@@ -339,21 +352,4 @@ static inline bool percpu_ref_is_dying(struct percpu_ref *ref)
 	return ref->percpu_count_ptr & __PERCPU_REF_DEAD;
 }
 
-/**
- * percpu_ref_is_zero - test whether a percpu refcount reached zero
- * @ref: percpu_ref to test
- *
- * Returns %true if @ref reached zero.
- *
- * This function is safe to call as long as @ref is between init and exit.
- */
-static inline bool percpu_ref_is_zero(struct percpu_ref *ref)
-{
-	unsigned long __percpu *percpu_count;
-
-	if (__ref_is_percpu(ref, &percpu_count))
-		return false;
-	return !atomic_long_read(&ref->count);
-}
-
 #endif