From: Ben Pfaff <blp@nicira.com>
Date: Fri, 1 May 2009 17:20:21 +0000 (-0700)
Subject: datapath: Eliminate synchronize_rcu() in table swap.
X-Git-Url: https://pintos-os.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=08343b78a89a16c3ac387d64900d2dc390a74414;p=openvswitch

datapath: Eliminate synchronize_rcu() in table swap.

We found out some time ago that synchronize_rcu() can block for multiple
seconds in some cases, so it's a good idea to eliminate as many of them
as we can.

This commit eliminates a call to synchronize_rcu() from functions that
expand or flush flow tables.  To avoid adding a member to dp_table that
specifies the "free_flows" argument to dp_table_destroy(), the commit
uses two different callback functions and manually inlines dp_table_swap()
into its callers.

Bug #1233.
---

diff --git a/datapath/datapath.h b/datapath/datapath.h
index 43914263..bb9ce0a3 100644
--- a/datapath/datapath.h
+++ b/datapath/datapath.h
@@ -34,6 +34,7 @@ struct sk_buff;
 struct dp_table {
 	unsigned int n_buckets;
 	struct sw_flow ***flows[2];
+	struct rcu_head rcu;
 };
 
 #define DP_N_QUEUES 2
diff --git a/datapath/table.c b/datapath/table.c
index b78f3b2f..7890442d 100644
--- a/datapath/table.c
+++ b/datapath/table.c
@@ -127,15 +127,6 @@ struct sw_flow *dp_table_lookup(struct dp_table *table,
 	return flow;
 }
 
-static void dp_table_swap(struct datapath *dp, struct dp_table *new_table,
-			  int free_flows)
-{
-	struct dp_table *old_table = rcu_dereference(dp->table);
-	rcu_assign_pointer(dp->table, new_table);
-	synchronize_rcu();
-	dp_table_destroy(old_table, free_flows);
-}
-
 int dp_table_foreach(struct dp_table *table,
 		     int (*callback)(struct sw_flow *flow, void *aux),
 		     void *aux)
@@ -174,6 +165,12 @@ static int insert_flow(struct sw_flow *flow, void *new_table_)
 	return 0;
 }
 
+static void dp_free_table_rcu(struct rcu_head *rcu)
+{
+	struct dp_table *table = container_of(rcu, struct dp_table, rcu);
+	dp_table_destroy(table, 0);
+}
+
 int dp_table_expand(struct datapath *dp)
 {
 	struct dp_table *old_table = rcu_dereference(dp->table);
@@ -181,16 +178,25 @@ int dp_table_expand(struct datapath *dp)
 	if (!new_table)
 		return -ENOMEM;
 	dp_table_foreach(old_table, insert_flow, new_table);
-	dp_table_swap(dp, new_table, 0);
+	rcu_assign_pointer(dp->table, new_table);
+	call_rcu(&old_table->rcu, dp_free_table_rcu);
 	return 0;
 }
 
+static void dp_free_table_and_flows_rcu(struct rcu_head *rcu)
+{
+	struct dp_table *table = container_of(rcu, struct dp_table, rcu);
+	dp_table_destroy(table, 1);
+}
+
 int dp_table_flush(struct datapath *dp)
 {
+	struct dp_table *old_table = rcu_dereference(dp->table);
 	struct dp_table *new_table = dp_table_create(DP_L1_SIZE);
 	if (!new_table)
 		return -ENOMEM;
-	dp_table_swap(dp, new_table, 1);
+	rcu_assign_pointer(dp->table, new_table);
+	call_rcu(&old_table->rcu, dp_free_table_and_flows_rcu);
 	return 0;
 }