Subject: [PATCH] net/fq_impl: do not maintain a backlog-sorted list of flows
From: Felix Fietkau <[email-masked]>
Date: Wed, 25 Nov 2020 18:10:34 +0100

A sorted flow list is only needed to drop packets in the biggest flow when
hitting the overmemory condition.
By scanning flows only when needed, we can avoid paying the cost of
maintaining the list under normal conditions
In order to avoid scanning lots of empty flows and touching too many cold
cache lines, a bitmap of flows with backlog is maintained

Signed-off-by: Felix Fietkau <[email-masked]>
---

--- a/include/net/fq.h
+++ b/include/net/fq.h
@@ -19,8 +19,6 @@ struct fq_tin;
  * @flowchain: can be linked to fq_tin's new_flows or old_flows. Used for DRR++
  *	(deficit round robin) based round robin queuing similar to the one
  *	found in net/sched/sch_fq_codel.c
- * @backlogchain: can be linked to other fq_flow and fq. Used to keep track of
- *	fat flows and efficient head-dropping if packet limit is reached
  * @queue: sk_buff queue to hold packets
  * @backlog: number of bytes pending in the queue. The number of packets can be
  *	found in @queue.qlen
@@ -29,7 +27,6 @@ struct fq_tin;
 struct fq_flow {
 	struct fq_tin *tin;
 	struct list_head flowchain;
-	struct list_head backlogchain;
 	struct sk_buff_head queue;
 	u32 backlog;
 	int deficit;
@@ -47,6 +44,7 @@ struct fq_flow {
 struct fq_tin {
 	struct list_head new_flows;
 	struct list_head old_flows;
+	struct list_head tin_list;
 	struct fq_flow default_flow;
 	u32 backlog_bytes;
 	u32 backlog_packets;
@@ -60,14 +58,14 @@ struct fq_tin {
 /**
  * struct fq - main container for fair queuing purposes
  *
- * @backlogs: linked to fq_flows. Used to maintain fat flows for efficient
- *	head-dropping when @backlog reaches @limit
  * @limit: max number of packets that can be queued across all flows
  * @backlog: number of packets queued across all flows
  */
 struct fq {
 	struct fq_flow *flows;
-	struct list_head backlogs;
+	unsigned long *flows_bitmap;
+
+	struct list_head tin_backlog;
 	spinlock_t lock;
 	u32 flows_cnt;
 	u32 limit;
--- a/include/net/fq_impl.h
+++ b/include/net/fq_impl.h
@@ -17,12 +17,24 @@ __fq_adjust_removal(struct fq *fq, struc
 		    unsigned int bytes, unsigned int truesize)
 {
 	struct fq_tin *tin = flow->tin;
+	int idx;
 
 	tin->backlog_bytes -= bytes;
 	tin->backlog_packets -= packets;
 	flow->backlog -= bytes;
 	fq->backlog -= packets;
 	fq->memory_usage -= truesize;
+
+	if (flow->backlog)
+		return;
+
+	if (flow == &tin->default_flow) {
+		list_del_init(&tin->tin_list);
+		return;
+	}
+
+	idx = flow - fq->flows;
+	__clear_bit(idx, fq->flows_bitmap);
 }
 
 static void fq_adjust_removal(struct fq *fq,
@@ -32,24 +44,6 @@ static void fq_adjust_removal(struct fq
 	__fq_adjust_removal(fq, flow, 1, skb->len, skb->truesize);
 }
 
-static void fq_rejigger_backlog(struct fq *fq, struct fq_flow *flow)
-{
-	struct fq_flow *i;
-
-	if (flow->backlog == 0) {
-		list_del_init(&flow->backlogchain);
-	} else {
-		i = flow;
-
-		list_for_each_entry_continue(i, &fq->backlogs, backlogchain)
-			if (i->backlog < flow->backlog)
-				break;
-
-		list_move_tail(&flow->backlogchain,
-			       &i->backlogchain);
-	}
-}
-
 static struct sk_buff *fq_flow_dequeue(struct fq *fq,
 				       struct fq_flow *flow)
 {
@@ -62,7 +56,6 @@ static struct sk_buff *fq_flow_dequeue(s
 		return NULL;
 
 	fq_adjust_removal(fq, flow, skb);
-	fq_rejigger_backlog(fq, flow);
 
 	return skb;
 }
@@ -90,7 +83,6 @@ static int fq_flow_drop(struct fq *fq, s
 	} while (packets < pending);
 
 	__fq_adjust_removal(fq, flow, packets, bytes, truesize);
-	fq_rejigger_backlog(fq, flow);
 
 	return packets;
 }
@@ -170,22 +162,36 @@ static struct fq_flow *fq_flow_classify(
 	return flow;
 }
 
-static void fq_recalc_backlog(struct fq *fq,
-			      struct fq_tin *tin,
-			      struct fq_flow *flow)
-{
-	struct fq_flow *i;
-
-	if (list_empty(&flow->backlogchain))
-		list_add_tail(&flow->backlogchain, &fq->backlogs);
-
-	i = flow;
-	list_for_each_entry_continue_reverse(i, &fq->backlogs,
-					     backlogchain)
-		if (i->backlog > flow->backlog)
-			break;
+static struct fq_flow *fq_find_fattest_flow(struct fq *fq)
+{
+	struct fq_tin *tin;
+	struct fq_flow *flow = NULL;
+	u32 len = 0;
+	int i;
+
+	for_each_set_bit(i, fq->flows_bitmap, fq->flows_cnt) {
+		struct fq_flow *cur = &fq->flows[i];
+		unsigned int cur_len;
+
+		cur_len = cur->backlog;
+		if (cur_len <= len)
+			continue;
+
+		flow = cur;
+		len = cur_len;
+	}
 
-	list_move(&flow->backlogchain, &i->backlogchain);
+	list_for_each_entry(tin, &fq->tin_backlog, tin_list) {
+		unsigned int cur_len = tin->default_flow.backlog;
+
+		if (cur_len <= len)
+			continue;
+
+		flow = &tin->default_flow;
+		len = cur_len;
+	}
+
+	return flow;
 }
 
 static void fq_tin_enqueue(struct fq *fq,
@@ -200,6 +206,13 @@ static void fq_tin_enqueue(struct fq *fq
 
 	flow = fq_flow_classify(fq, tin, idx, skb);
 
+	if (!flow->backlog) {
+		if (flow != &tin->default_flow)
+			__set_bit(idx, fq->flows_bitmap);
+		else if (list_empty(&tin->tin_list))
+			list_add(&tin->tin_list, &fq->tin_backlog);
+	}
+
 	flow->tin = tin;
 	flow->backlog += skb->len;
 	tin->backlog_bytes += skb->len;
@@ -207,8 +220,6 @@ static void fq_tin_enqueue(struct fq *fq
 	fq->memory_usage += skb->truesize;
 	fq->backlog++;
 
-	fq_recalc_backlog(fq, tin, flow);
-
 	if (list_empty(&flow->flowchain)) {
 		flow->deficit = fq->quantum;
 		list_add_tail(&flow->flowchain,
@@ -218,9 +229,7 @@ static void fq_tin_enqueue(struct fq *fq
 	__skb_queue_tail(&flow->queue, skb);
 	oom = (fq->memory_usage > fq->memory_limit);
 	while (fq->backlog > fq->limit || oom) {
-		flow = list_first_entry_or_null(&fq->backlogs,
-						struct fq_flow,
-						backlogchain);
+		flow = fq_find_fattest_flow(fq);
 		if (!flow)
 			return;
 
@@ -255,8 +264,6 @@ static void fq_flow_filter(struct fq *fq
 		fq_adjust_removal(fq, flow, skb);
 		free_func(fq, tin, flow, skb);
 	}
-
-	fq_rejigger_backlog(fq, flow);
 }
 
 static void fq_tin_filter(struct fq *fq,
@@ -279,16 +286,18 @@ static void fq_flow_reset(struct fq *fq,
 			  struct fq_flow *flow,
 			  fq_skb_free_t free_func)
 {
+	struct fq_tin *tin = flow->tin;
 	struct sk_buff *skb;
 
 	while ((skb = fq_flow_dequeue(fq, flow)))
-		free_func(fq, flow->tin, flow, skb);
+		free_func(fq, tin, flow, skb);
 
-	if (!list_empty(&flow->flowchain))
+	if (!list_empty(&flow->flowchain)) {
 		list_del_init(&flow->flowchain);
-
-	if (!list_empty(&flow->backlogchain))
-		list_del_init(&flow->backlogchain);
+		if (list_empty(&tin->new_flows) &&
+		    list_empty(&tin->old_flows))
+			list_del_init(&tin->tin_list);
+	}
 
 	flow->tin = NULL;
 
@@ -314,6 +323,7 @@ static void fq_tin_reset(struct fq *fq,
 		fq_flow_reset(fq, flow, free_func);
 	}
 
+	WARN_ON_ONCE(!list_empty(&tin->tin_list));
 	WARN_ON_ONCE(tin->backlog_bytes);
 	WARN_ON_ONCE(tin->backlog_packets);
 }
@@ -321,7 +331,6 @@ static void fq_tin_reset(struct fq *fq,
 static void fq_flow_init(struct fq_flow *flow)
 {
 	INIT_LIST_HEAD(&flow->flowchain);
-	INIT_LIST_HEAD(&flow->backlogchain);
 	__skb_queue_head_init(&flow->queue);
 }
 
@@ -329,6 +338,7 @@ static void fq_tin_init(struct fq_tin *t
 {
 	INIT_LIST_HEAD(&tin->new_flows);
 	INIT_LIST_HEAD(&tin->old_flows);
+	INIT_LIST_HEAD(&tin->tin_list);
 	fq_flow_init(&tin->default_flow);
 }
 
@@ -337,8 +347,8 @@ static int fq_init(struct fq *fq, int fl
 	int i;
 
 	memset(fq, 0, sizeof(fq[0]));
-	INIT_LIST_HEAD(&fq->backlogs);
 	spin_lock_init(&fq->lock);
+	INIT_LIST_HEAD(&fq->tin_backlog);
 	fq->flows_cnt = max_t(u32, flows_cnt, 1);
 	fq->quantum = 300;
 	fq->limit = 8192;
@@ -348,6 +358,14 @@ static int fq_init(struct fq *fq, int fl
 	if (!fq->flows)
 		return -ENOMEM;
 
+	fq->flows_bitmap = kcalloc(BITS_TO_LONGS(fq->flows_cnt), sizeof(long),
+				   GFP_KERNEL);
+	if (!fq->flows_bitmap) {
+		kvfree(fq->flows);
+		fq->flows = NULL;
+		return -ENOMEM;
+	}
+
 	for (i = 0; i < fq->flows_cnt; i++)
 		fq_flow_init(&fq->flows[i]);
 
@@ -364,6 +382,9 @@ static void fq_reset(struct fq *fq,
 
 	kvfree(fq->flows);
 	fq->flows = NULL;
+
+	kfree(fq->flows_bitmap);
+	fq->flows_bitmap = NULL;
 }
 
 #endif
--- a/net/mac80211/tx.c
+++ b/net/mac80211/tx.c
@@ -3364,8 +3364,6 @@ out_recalc:
 	if (head->len != orig_len) {
 		flow->backlog += head->len - orig_len;
 		tin->backlog_bytes += head->len - orig_len;
-
-		fq_recalc_backlog(fq, tin, flow);
 	}
 out:
 	spin_unlock_bh(&fq->lock);