srcu: Begin offloading srcu_struct fields to srcu_update

The current srcu_struct structure is on the order of 200 bytes in size
(depending on architecture and .config), which is much better than the
old-style 26K bytes, but still all too inconvenient when one is trying
to achieve good cache locality on a fastpath involving SRCU readers.

However, only a few fields in srcu_struct are used by SRCU readers.
The remaining fields could be offloaded to a new srcu_update
structure, thus shrinking the srcu_struct structure down to a few
tens of bytes.  This commit begins this noble quest, a quest that is
complicated by open-coded initialization of the srcu_struct within the
srcu_notifier_head structure.  This complication is addressed by updating
the srcu_notifier_head structure's open coding, given that there does
not appear to be a straightforward way of abstracting that initialization.

This commit moves only the ->node pointer to srcu_update.  Later commits
will move additional fields.

[ paulmck: Fold in qiang1.zhang@intel.com's memory-leak fix. ]

Link: https://lore.kernel.org/all/20230320055751.4120251-1-qiang1.zhang@intel.com/
Suggested-by: Christoph Hellwig <hch@lst.de>
Cc: "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>
Cc: "Michał Mirosław" <mirq-linux@rere.qmqm.pl>
Cc: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Tested-by: Sachin Sant <sachinp@linux.ibm.com>
Tested-by: "Zhang, Qiang1" <qiang1.zhang@intel.com>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Tested-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

1 files changed, 18 insertions, 9 deletions

diff --git a/include/linux/srcutree.h b/include/linux/srcutree.h
index 3ce6deee1dbe..276f325f1296 100644
--- a/include/linux/srcutree.h
+++ b/include/linux/srcutree.h
@@ -58,10 +58,16 @@ struct srcu_node {
 };
 
 /*
+ * Per-SRCU-domain structure, update-side data linked from srcu_struct.
+ */
+struct srcu_usage {
+	struct srcu_node *node;			/* Combining tree. */
+};
+
+/*
  * Per-SRCU-domain structure, similar in function to rcu_state.
  */
 struct srcu_struct {
-	struct srcu_node *node;			/* Combining tree. */
 	struct srcu_node *level[RCU_NUM_LVLS + 1];
 						/* First node at each level. */
 	int srcu_size_state;			/* Small-to-big transition state. */
@@ -90,6 +96,7 @@ struct srcu_struct {
 	unsigned long reschedule_count;
 	struct delayed_work work;
 	struct lockdep_map dep_map;
+	struct srcu_usage *srcu_sup;		/* Update-side data. */
 };
 
 /* Values for size state variable (->srcu_size_state). */
@@ -108,24 +115,24 @@ struct srcu_struct {
 #define SRCU_STATE_SCAN1	1
 #define SRCU_STATE_SCAN2	2
 
-#define __SRCU_STRUCT_INIT_COMMON(name)								\
+#define __SRCU_STRUCT_INIT_COMMON(name, usage_name)						\
 	.lock = __SPIN_LOCK_UNLOCKED(name.lock),						\
 	.srcu_gp_seq_needed = -1UL,								\
 	.work = __DELAYED_WORK_INITIALIZER(name.work, NULL, 0),					\
+	.srcu_sup = &usage_name,								\
 	__SRCU_DEP_MAP_INIT(name)
 
-#define __SRCU_STRUCT_INIT_MODULE(name)								\
+#define __SRCU_STRUCT_INIT_MODULE(name, usage_name)						\
 {												\
-	__SRCU_STRUCT_INIT_COMMON(name)								\
+	__SRCU_STRUCT_INIT_COMMON(name, usage_name)						\
 }
 
-#define __SRCU_STRUCT_INIT(name, pcpu_name)							\
+#define __SRCU_STRUCT_INIT(name, usage_name, pcpu_name)						\
 {												\
 	.sda = &pcpu_name,									\
-	__SRCU_STRUCT_INIT_COMMON(name)								\
+	__SRCU_STRUCT_INIT_COMMON(name, usage_name)						\
 }
 
-
 /*
  * Define and initialize a srcu struct at build time.
  * Do -not- call init_srcu_struct() nor cleanup_srcu_struct() on it.
@@ -147,15 +154,17 @@ struct srcu_struct {
  */
 #ifdef MODULE
 # define __DEFINE_SRCU(name, is_static)								\
-	is_static struct srcu_struct name = __SRCU_STRUCT_INIT_MODULE(name);			\
+	static struct srcu_usage name##_srcu_usage;						\
+	is_static struct srcu_struct name = __SRCU_STRUCT_INIT_MODULE(name, name##_srcu_usage);	\
 	extern struct srcu_struct * const __srcu_struct_##name;					\
 	struct srcu_struct * const __srcu_struct_##name						\
 		__section("___srcu_struct_ptrs") = &name
 #else
 # define __DEFINE_SRCU(name, is_static)								\
 	static DEFINE_PER_CPU(struct srcu_data, name##_srcu_data);				\
+	static struct srcu_usage name##_srcu_usage;						\
 	is_static struct srcu_struct name =							\
-		__SRCU_STRUCT_INIT(name, name##_srcu_data)
+		__SRCU_STRUCT_INIT(name, name##_srcu_usage, name##_srcu_data)
 #endif
 #define DEFINE_SRCU(name)		__DEFINE_SRCU(name, /* not static */)
 #define DEFINE_STATIC_SRCU(name)	__DEFINE_SRCU(name, static)