[RFC PATCH v2 1/2] percpu_counter: introduce atomic mode for percpu_counter

[Peng Zhang <zhangpeng362@huawei.com>]

From: ZhangPeng <zhangpeng362@huawei.com>

Depending on whether counters is NULL, we can support two modes:
atomic mode and perpcu mode. We implement both modes by grouping
the s64 count and atomic64_t count_atomic in a union. At the same time,
we create the interface for adding and reading in atomic mode and for
switching atomic mode to percpu mode.

Suggested-by: Jan Kara <jack@suse.cz>
Signed-off-by: ZhangPeng <zhangpeng362@huawei.com>
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
---
 include/linux/percpu_counter.h | 43 +++++++++++++++++++++++++++++++---
 lib/percpu_counter.c           | 31 ++++++++++++++++++++++--
 2 files changed, 69 insertions(+), 5 deletions(-)

diff --git a/include/linux/percpu_counter.h b/include/linux/percpu_counter.h
index 3a44dd1e33d2..160f9734c0bb 100644
--- a/include/linux/percpu_counter.h
+++ b/include/linux/percpu_counter.h
@@ -21,7 +21,13 @@
 
 struct percpu_counter {
 	raw_spinlock_t lock;
-	s64 count;
+	/* Depending on whether counters is NULL, we can support two modes,
+	 * atomic mode using count_atomic and perpcu mode using count.
+	 */
+	union {
+		s64 count;
+		atomic64_t count_atomic;
+	};
 #ifdef CONFIG_HOTPLUG_CPU
 	struct list_head list;	/* All percpu_counters are on a list */
 #endif
@@ -32,14 +38,14 @@ extern int percpu_counter_batch;
 
 int __percpu_counter_init_many(struct percpu_counter *fbc, s64 amount,
 			       gfp_t gfp, u32 nr_counters,
-			       struct lock_class_key *key);
+			       struct lock_class_key *key, bool switch_mode);
 
 #define percpu_counter_init_many(fbc, value, gfp, nr_counters)		\
 	({								\
 		static struct lock_class_key __key;			\
 									\
 		__percpu_counter_init_many(fbc, value, gfp, nr_counters,\
-					   &__key);			\
+					   &__key, false);		\
 	})
 
 
@@ -130,6 +136,20 @@ static inline bool percpu_counter_initialized(struct percpu_counter *fbc)
 	return (fbc->counters != NULL);
 }
 
+static inline s64 percpu_counter_atomic_read(struct percpu_counter *fbc)
+{
+	return atomic64_read(&fbc->count_atomic);
+}
+
+static inline void percpu_counter_atomic_add(struct percpu_counter *fbc,
+					     s64 amount)
+{
+	atomic64_add(amount, &fbc->count_atomic);
+}
+
+int percpu_counter_switch_to_pcpu_many(struct percpu_counter *fbc,
+				       u32 nr_counters);
+
 #else /* !CONFIG_SMP */
 
 struct percpu_counter {
@@ -260,6 +280,23 @@ static inline bool percpu_counter_initialized(struct percpu_counter *fbc)
 static inline void percpu_counter_sync(struct percpu_counter *fbc)
 {
 }
+
+static inline s64 percpu_counter_atomic_read(struct percpu_counter *fbc)
+{
+	return fbc->count;
+}
+
+static inline void percpu_counter_atomic_add(struct percpu_counter *fbc,
+					     s64 amount)
+{
+	percpu_counter_add(fbc, amount);
+}
+
+static inline int percpu_counter_switch_to_pcpu_many(struct percpu_counter *fbc,
+						     u32 nr_counters)
+{
+	return 0;
+}
 #endif	/* CONFIG_SMP */
 
 static inline void percpu_counter_inc(struct percpu_counter *fbc)
diff --git a/lib/percpu_counter.c b/lib/percpu_counter.c
index 44dd133594d4..95c4e038051a 100644
--- a/lib/percpu_counter.c
+++ b/lib/percpu_counter.c
@@ -153,7 +153,7 @@ EXPORT_SYMBOL(__percpu_counter_sum);
 
 int __percpu_counter_init_many(struct percpu_counter *fbc, s64 amount,
 			       gfp_t gfp, u32 nr_counters,
-			       struct lock_class_key *key)
+			       struct lock_class_key *key, bool switch_mode)
 {
 	unsigned long flags __maybe_unused;
 	size_t counter_size;
@@ -174,7 +174,8 @@ int __percpu_counter_init_many(struct percpu_counter *fbc, s64 amount,
 #ifdef CONFIG_HOTPLUG_CPU
 		INIT_LIST_HEAD(&fbc[i].list);
 #endif
-		fbc[i].count = amount;
+		if (likely(!switch_mode))
+			fbc[i].count = amount;
 		fbc[i].counters = (void *)counters + (i * counter_size);
 
 		debug_percpu_counter_activate(&fbc[i]);
@@ -357,6 +358,32 @@ bool __percpu_counter_limited_add(struct percpu_counter *fbc,
 	return good;
 }
 
+/*
+ * percpu_counter_switch_to_pcpu_many: Converts struct percpu_counters from
+ * atomic mode to percpu mode.
+ */
+int percpu_counter_switch_to_pcpu_many(struct percpu_counter *fbc,
+				       u32 nr_counters)
+{
+	static struct lock_class_key __key;
+	unsigned long flags;
+	bool ret = 0;
+
+	if (percpu_counter_initialized(fbc))
+		return 0;
+
+	preempt_disable();
+	local_irq_save(flags);
+	if (likely(!percpu_counter_initialized(fbc)))
+		ret = __percpu_counter_init_many(fbc, 0,
+					GFP_ATOMIC|__GFP_NOWARN|__GFP_ZERO,
+					nr_counters, &__key, true);
+	local_irq_restore(flags);
+	preempt_enable();
+
+	return ret;
+}
+
 static int __init percpu_counter_startup(void)
 {
 	int ret;
-- 
2.25.1