[PATCH] net: openvswitch: reduce cpu_used_mask memory consumption

[taoyuan_eddy@hotmail.com]

From: eddytaoyuan <taoyuan_eddy@hotmail.com>

struct cpumask cpu_used_mask is directly embedded in struct sw_flow
however, its size is hardcoded to CONFIG_NR_CPUS bits, which
can be as large as 8192 by default, it cost memory and slows down
ovs_flow_alloc, this fix used actual CPU number instead

Signed-off-by: eddytaoyuan <taoyuan_eddy@hotmail.com>
---
 net/openvswitch/flow.c       |  6 +++---
 net/openvswitch/flow.h       |  2 +-
 net/openvswitch/flow_table.c | 25 ++++++++++++++++++++++---
 3 files changed, 26 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..c0fdff73272f 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 +1195,27 @@ 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 including 'struct cpumask' in 'struct sw_flow'
+         * consumes memory unnecessarily large.
+         * The reason is that compilation option CONFIG_NR_CPUS(default 8192)
+         * decides the value of NR_CPUS, which in turn decides size of
+         * 'struct cpumask', which means 1024 bytes are needed for the cpumask
+         * It affects ovs_flow_alloc performance as well as memory footprint.
+         * 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 actual memory
+         * of struct cpumask
+         *
+         * cpumask APIs like cpumask_next and cpumask_set_cpu have been defined
+         * to never access bits beyond cpu count by design, thus 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