[PATCH rcu 08/19] srcu: Add boot-time control over srcu_node array allocation

["Paul E. McKenney" <paulmck@kernel.org>]

This commit adds an srcu_tree.convert_to_big kernel parameter that either
refuses to convert at all (0), converts immediately at init_srcu_struct()
time (1), or lets rcutorture convert it (2).  An addition contention-based
dynamic convertion choice will be added, along with documentation.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
 kernel/rcu/srcutree.c | 27 ++++++++++++++++++++++++---
 1 file changed, 24 insertions(+), 3 deletions(-)

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 550991cc213d3..d1f1ff930acf5 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -39,6 +39,15 @@ module_param(exp_holdoff, ulong, 0444);
 static ulong counter_wrap_check = (ULONG_MAX >> 2);
 module_param(counter_wrap_check, ulong, 0444);
 
+/*
+ * Control conversion to SRCU_SIZE_BIG:
+ * 0: Don't convert at all (default).
+ * 1: Convert at init_srcu_struct() time.
+ * 2: Convert when rcutorture invokes srcu_torture_stats_print().
+ */
+static int convert_to_big;
+module_param(convert_to_big, int, 0444);
+
 /* Early-boot callback-management, so early that no lock is required! */
 static LIST_HEAD(srcu_boot_list);
 static bool __read_mostly srcu_init_done;
@@ -113,7 +122,7 @@ static void init_srcu_struct_data(struct srcu_struct *ssp)
  * Allocated and initialize SRCU combining tree.  Returns @true if
  * allocation succeeded and @false otherwise.
  */
-static bool init_srcu_struct_nodes(struct srcu_struct *ssp)
+static bool init_srcu_struct_nodes(struct srcu_struct *ssp, gfp_t gfp_flags)
 {
 	int cpu;
 	int i;
@@ -125,7 +134,7 @@ static bool init_srcu_struct_nodes(struct srcu_struct *ssp)
 
 	/* Initialize geometry if it has not already been initialized. */
 	rcu_init_geometry();
-	ssp->node = kcalloc(rcu_num_nodes, sizeof(*ssp->node), GFP_ATOMIC);
+	ssp->node = kcalloc(rcu_num_nodes, sizeof(*ssp->node), gfp_flags);
 	if (!ssp->node)
 		return false;
 
@@ -204,6 +213,16 @@ static int init_srcu_struct_fields(struct srcu_struct *ssp, bool is_static)
 	init_srcu_struct_data(ssp);
 	ssp->srcu_gp_seq_needed_exp = 0;
 	ssp->srcu_last_gp_end = ktime_get_mono_fast_ns();
+	if (READ_ONCE(ssp->srcu_size_state) == SRCU_SIZE_SMALL && convert_to_big == 1) {
+		if (!init_srcu_struct_nodes(ssp, GFP_ATOMIC)) {
+			if (!is_static) {
+				free_percpu(ssp->sda);
+				ssp->sda = NULL;
+			}
+			return -ENOMEM;
+		}
+		WRITE_ONCE(ssp->srcu_size_state, SRCU_SIZE_BIG);
+	}
 	smp_store_release(&ssp->srcu_gp_seq_needed, 0); /* Init done. */
 	return 0;
 }
@@ -627,7 +646,7 @@ static void srcu_gp_end(struct srcu_struct *ssp)
 	ss_state = smp_load_acquire(&ssp->srcu_size_state);
 	if (ss_state && ss_state != SRCU_SIZE_BIG) {
 		if (ss_state == SRCU_SIZE_ALLOC)
-			init_srcu_struct_nodes(ssp);
+			init_srcu_struct_nodes(ssp, GFP_KERNEL);
 		else
 			smp_store_release(&ssp->srcu_size_state, ss_state + 1);
 	}
@@ -1456,6 +1475,8 @@ void srcu_torture_stats_print(struct srcu_struct *ssp, char *tt, char *tf)
 		s1 += c1;
 	}
 	pr_cont(" T(%ld,%ld)\n", s0, s1);
+	if (READ_ONCE(ssp->srcu_size_state) == SRCU_SIZE_SMALL && convert_to_big == 2)
+		WRITE_ONCE(ssp->srcu_size_state, SRCU_SIZE_ALLOC);
 }
 EXPORT_SYMBOL_GPL(srcu_torture_stats_print);
 
-- 
2.31.1.189.g2e36527f23