[PATCH v2 1/1] net: openvswitch: reduce cpu_used_mask memory

[taoyuan_eddy@hotmail.com]

From: Eddy Tao <taoyuan_eddy@hotmail.com>

'struct cpumask cpu_used_mask' is embedded in struct sw_flow.
However, its size is hardcoded to CONFIG_NR_CPUS bits, which can be
8192 by default, it costs memory and slows down ovs_flow_alloc.
This fix uses actual CPU number instead

Signed-off-by: Eddy Tao <taoyuan_eddy@hotmail.com>
---
 net/openvswitch/flow.c       |  6 +++---
 net/openvswitch/flow.h       |  2 +-
 net/openvswitch/flow_table.c | 24 +++++++++++++++++++++---
 3 files changed, 25 insertions(+), 7 deletions(-)

diff --git a/net/openvswitch/flow.c b/net/openvswitch/flow.c
index e20d1a973417..06345cd8c777 100644
--- a/net/openvswitch/flow.c
+++ b/net/openvswitch/flow.c
@@ -107,7 +107,7 @@ void ovs_flow_stats_update(struct sw_flow *flow, __be16 tcp_flags,
 
 					rcu_assign_pointer(flow->stats[cpu],
 							   new_stats);
-					cpumask_set_cpu(cpu, &flow->cpu_used_mask);
+					cpumask_set_cpu(cpu, flow->cpu_used_mask);
 					goto unlock;
 				}
 			}
@@ -135,7 +135,7 @@ void ovs_flow_stats_get(const struct sw_flow *flow,
 	memset(ovs_stats, 0, sizeof(*ovs_stats));
 
 	/* We open code this to make sure cpu 0 is always considered */
-	for (cpu = 0; cpu < nr_cpu_ids; cpu = cpumask_next(cpu, &flow->cpu_used_mask)) {
+	for (cpu = 0; cpu < nr_cpu_ids; cpu = cpumask_next(cpu, flow->cpu_used_mask)) {
 		struct sw_flow_stats *stats = rcu_dereference_ovsl(flow->stats[cpu]);
 
 		if (stats) {
@@ -159,7 +159,7 @@ void ovs_flow_stats_clear(struct sw_flow *flow)
 	int cpu;
 
 	/* We open code this to make sure cpu 0 is always considered */
-	for (cpu = 0; cpu < nr_cpu_ids; cpu = cpumask_next(cpu, &flow->cpu_used_mask)) {
+	for (cpu = 0; cpu < nr_cpu_ids; cpu = cpumask_next(cpu, flow->cpu_used_mask)) {
 		struct sw_flow_stats *stats = ovsl_dereference(flow->stats[cpu]);
 
 		if (stats) {
diff --git a/net/openvswitch/flow.h b/net/openvswitch/flow.h
index 073ab73ffeaa..b5711aff6e76 100644
--- a/net/openvswitch/flow.h
+++ b/net/openvswitch/flow.h
@@ -229,7 +229,7 @@ struct sw_flow {
 					 */
 	struct sw_flow_key key;
 	struct sw_flow_id id;
-	struct cpumask cpu_used_mask;
+	struct cpumask *cpu_used_mask;
 	struct sw_flow_mask *mask;
 	struct sw_flow_actions __rcu *sf_acts;
 	struct sw_flow_stats __rcu *stats[]; /* One for each CPU.  First one
diff --git a/net/openvswitch/flow_table.c b/net/openvswitch/flow_table.c
index 0a0e4c283f02..63c95c9a814d 100644
--- a/net/openvswitch/flow_table.c
+++ b/net/openvswitch/flow_table.c
@@ -87,11 +87,12 @@ struct sw_flow *ovs_flow_alloc(void)
 	if (!stats)
 		goto err;
 
+	flow->cpu_used_mask = (struct cpumask *)&flow->stats[nr_cpu_ids];
 	spin_lock_init(&stats->lock);
 
 	RCU_INIT_POINTER(flow->stats[0], stats);
 
-	cpumask_set_cpu(0, &flow->cpu_used_mask);
+	cpumask_set_cpu(0, flow->cpu_used_mask);
 
 	return flow;
 err:
@@ -115,7 +116,7 @@ static void flow_free(struct sw_flow *flow)
 					  flow->sf_acts);
 	/* We open code this to make sure cpu 0 is always considered */
 	for (cpu = 0; cpu < nr_cpu_ids;
-	     cpu = cpumask_next(cpu, &flow->cpu_used_mask)) {
+	     cpu = cpumask_next(cpu, flow->cpu_used_mask)) {
 		if (flow->stats[cpu])
 			kmem_cache_free(flow_stats_cache,
 					(struct sw_flow_stats __force *)flow->stats[cpu]);
@@ -1194,9 +1195,26 @@ int ovs_flow_init(void)
 	BUILD_BUG_ON(__alignof__(struct sw_flow_key) % __alignof__(long));
 	BUILD_BUG_ON(sizeof(struct sw_flow_key) % sizeof(long));
 
+	/* Simply embedding 'struct cpumask' in 'struct sw_flow'
+	 * consumes memory unnecessarily large. Cpumask is an bitmap
+	 * of CONFIG_NR_CPUS bits, which is hardcoded in .config
+	 * and default value can be 8192, in this case is 1024 bytes.
+	 * It drops ovs_flow_alloc performance and cost memory.
+	 * We should use actual CPU count instead of hardcoded value.
+	 *
+	 * To address this, 'cpu_used_mask' is redefined to pointer
+	 * and append a cpumask_size() after 'stat' to hold the memory
+	 * for struct cpumask.
+	 *
+	 * cpumask APIs like cpumask_next and cpumask_set_cpu are defined
+	 * to never access bits beyond cpu count, as such above change is
+	 * safe even though the actual memory provided is smaller than
+	 * sizeof(struct cpumask)
+	 */
 	flow_cache = kmem_cache_create("sw_flow", sizeof(struct sw_flow)
 				       + (nr_cpu_ids
-					  * sizeof(struct sw_flow_stats *)),
+					  * sizeof(struct sw_flow_stats *))
+				       + cpumask_size(),
 				       0, 0, NULL);
 	if (flow_cache == NULL)
 		return -ENOMEM;
-- 
2.27.0