srcu: Make early-boot call_srcu() reuse workqueue lists

Allocating a list_head structure that is almost never used, and, when
used, is used only during early boot (rcu_init() and earlier), is a bit
wasteful.  This commit therefore eliminates that list_head in favor of
the one in the work_struct structure.  This is safe because the work_struct
structure cannot be used until after rcu_init() returns.

Reported-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Tested-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

include/linux/srcutiny.h

@@ -36,7 +36,6 @@ struct srcu_struct {
 	struct rcu_head *srcu_cb_head;	/* Pending callbacks: Head. */
 	struct rcu_head **srcu_cb_tail;	/* Pending callbacks: Tail. */
 	struct work_struct srcu_work;	/* For driving grace periods. */
-	struct list_head srcu_boot_entry; /* Early-boot callbacks. */
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 	struct lockdep_map dep_map;
 #endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
@@ -49,7 +48,6 @@ void srcu_drive_gp(struct work_struct *wp);
 	.srcu_wq = __SWAIT_QUEUE_HEAD_INITIALIZER(name.srcu_wq),	\
 	.srcu_cb_tail = &name.srcu_cb_head,				\
 	.srcu_work = __WORK_INITIALIZER(name.srcu_work, srcu_drive_gp),	\
-	.srcu_boot_entry = LIST_HEAD_INIT(name.srcu_boot_entry),	\
 	__SRCU_DEP_MAP_INIT(name)					\
 }
 

include/linux/srcutree.h

@@ -94,7 +94,6 @@ struct srcu_struct {
 						/*  callback for the barrier */
 						/*  operation. */
 	struct delayed_work work;
-	struct list_head srcu_boot_entry;	/* Early-boot callbacks. */
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 	struct lockdep_map dep_map;
 #endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
@@ -110,7 +109,7 @@ struct srcu_struct {
 	.sda = &pcpu_name,						\
 	.lock = __SPIN_LOCK_UNLOCKED(name.lock),			\
 	.srcu_gp_seq_needed = -1UL,					\
-	.srcu_boot_entry = LIST_HEAD_INIT(name.srcu_boot_entry),	\
+	.work = __DELAYED_WORK_INITIALIZER(name.work, NULL, 0),		\
 	__SRCU_DEP_MAP_INIT(name)					\
 }
 

kernel/rcu/srcutiny.c

@@ -48,7 +48,7 @@ static int init_srcu_struct_fields(struct srcu_struct *sp)
 	sp->srcu_gp_waiting = false;
 	sp->srcu_idx = 0;
 	INIT_WORK(&sp->srcu_work, srcu_drive_gp);
-	INIT_LIST_HEAD(&sp->srcu_boot_entry);
+	INIT_LIST_HEAD(&sp->srcu_work.entry);
 	return 0;
 }
 
@@ -185,8 +185,8 @@ void call_srcu(struct srcu_struct *sp, struct rcu_head *rhp,
 	if (!READ_ONCE(sp->srcu_gp_running)) {
 		if (likely(srcu_init_done))
 			schedule_work(&sp->srcu_work);
-		else if (list_empty(&sp->srcu_boot_entry))
-			list_add(&sp->srcu_boot_entry, &srcu_boot_list);
+		else if (list_empty(&sp->srcu_work.entry))
+			list_add(&sp->srcu_work.entry, &srcu_boot_list);
 	}
 }
 EXPORT_SYMBOL_GPL(call_srcu);
@@ -224,8 +224,8 @@ void __init srcu_init(void)
 	srcu_init_done = true;
 	while (!list_empty(&srcu_boot_list)) {
 		sp = list_first_entry(&srcu_boot_list,
-				      struct srcu_struct, srcu_boot_entry);
-		list_del_init(&sp->srcu_boot_entry);
+				      struct srcu_struct, srcu_work.entry);
+		list_del_init(&sp->srcu_work.entry);
 		schedule_work(&sp->srcu_work);
 	}
 }

kernel/rcu/srcutree.c

@@ -186,7 +186,6 @@ static int init_srcu_struct_fields(struct srcu_struct *sp, bool is_static)
 	mutex_init(&sp->srcu_barrier_mutex);
 	atomic_set(&sp->srcu_barrier_cpu_cnt, 0);
 	INIT_DELAYED_WORK(&sp->work, process_srcu);
-	INIT_LIST_HEAD(&sp->srcu_boot_entry);
 	if (!is_static)
 		sp->sda = alloc_percpu(struct srcu_data);
 	init_srcu_struct_nodes(sp, is_static);
@@ -708,8 +707,8 @@ static void srcu_funnel_gp_start(struct srcu_struct *sp, struct srcu_data *sdp,
 		if (likely(srcu_init_done))
 			queue_delayed_work(rcu_gp_wq, &sp->work,
 					   srcu_get_delay(sp));
-		else if (list_empty(&sp->srcu_boot_entry))
-			list_add(&sp->srcu_boot_entry, &srcu_boot_list);
+		else if (list_empty(&sp->work.work.entry))
+			list_add(&sp->work.work.entry, &srcu_boot_list);
 	}
 	spin_unlock_irqrestore_rcu_node(sp, flags);
 }
@@ -1323,10 +1322,10 @@ void __init srcu_init(void)
 
 	srcu_init_done = true;
 	while (!list_empty(&srcu_boot_list)) {
-		sp = list_first_entry(&srcu_boot_list,
-				      struct srcu_struct, srcu_boot_entry);
+		sp = list_first_entry(&srcu_boot_list, struct srcu_struct,
+				      work.work.entry);
 		check_init_srcu_struct(sp);
-		list_del_init(&sp->srcu_boot_entry);
+		list_del_init(&sp->work.work.entry);
 		queue_work(rcu_gp_wq, &sp->work.work);
 	}
 }