net: sk_buff rbnode reorg

skb->rbnode shares space with skb->next, skb->prev and skb->tstamp

Current uses (TCP receive ofo queue and netem) need to save/restore
tstamp, while skb->dev is either NULL (TCP) or a constant for a given
queue (netem).

Since we plan using an RB tree for TCP retransmit queue to speedup SACK
processing with large BDP, this patch exchanges skb->dev and
skb->tstamp.

This saves some overhead in both TCP and netem.

v2: removes the swtstamp field from struct tcp_skb_cb

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Soheil Hassas Yeganeh <soheil@google.com>
Cc: Wei Wang <weiwan@google.com>
Cc: Willem de Bruijn <willemb@google.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/skbuff.h

@@ -661,21 +661,21 @@ struct sk_buff {
  struct sk_buff *prev;
 
  union {
- ktime_t tstamp;
- u64 skb_mstamp;
+ struct net_device *dev;
+ /* Some protocols might use this space to store information,
+ * while device pointer would be NULL.
+ * UDP receive path is one user.
+ */
+ unsigned long dev_scratch;
  };
  };
  struct rb_node rbnode; /* used in netem & tcp stack */
  };
  struct sock *sk;
 
  union {
- struct net_device *dev;
- /* Some protocols might use this space to store information,
- * while device pointer would be NULL.
- * UDP receive path is one user.
- */
- unsigned long dev_scratch;
+ ktime_t tstamp;
+ u64 skb_mstamp;
  };
  /*
  * This is the control buffer. It is free to use for every

include/net/tcp.h

@@ -797,12 +797,6 @@ struct tcp_skb_cb {
  u16 tcp_gso_segs;
  u16 tcp_gso_size;
  };
-
- /* Used to stash the receive timestamp while this skb is in the
- * out of order queue, as skb->tstamp is overwritten by the
- * rbnode.
- */
- ktime_t swtstamp;
  };
  __u8 tcp_flags; /* TCP header flags. (tcp[13]) */
 

net/ipv4/tcp_input.c

@@ -4266,11 +4266,6 @@ static void tcp_sack_remove(struct tcp_sock *tp)
  tp->rx_opt.num_sacks = num_sacks;
 }
 
-enum tcp_queue {
- OOO_QUEUE,
- RCV_QUEUE,
-};
-
 /**
  * tcp_try_coalesce - try to merge skb to prior one
  * @sk: socket
@@ -4286,7 +4281,6 @@ enum tcp_queue {
  * Returns true if caller should free @from instead of queueing it
  */
 static bool tcp_try_coalesce(struct sock *sk,
- enum tcp_queue dest,
  struct sk_buff *to,
  struct sk_buff *from,
  bool *fragstolen)
@@ -4311,10 +4305,7 @@ static bool tcp_try_coalesce(struct sock *sk,
 
  if (TCP_SKB_CB(from)->has_rxtstamp) {
  TCP_SKB_CB(to)->has_rxtstamp = true;
- if (dest == OOO_QUEUE)
- TCP_SKB_CB(to)->swtstamp = TCP_SKB_CB(from)->swtstamp;
- else
- to->tstamp = from->tstamp;
+ to->tstamp = from->tstamp;
  }
 
  return true;
@@ -4351,9 +4342,6 @@ static void tcp_ofo_queue(struct sock *sk)
  }
  p = rb_next(p);
  rb_erase(&skb->rbnode, &tp->out_of_order_queue);
- /* Replace tstamp which was stomped by rbnode */
- if (TCP_SKB_CB(skb)->has_rxtstamp)
- skb->tstamp = TCP_SKB_CB(skb)->swtstamp;
 
  if (unlikely(!after(TCP_SKB_CB(skb)->end_seq, tp->rcv_nxt))) {
  SOCK_DEBUG(sk, "ofo packet was already received\n");
@@ -4365,8 +4353,7 @@ static void tcp_ofo_queue(struct sock *sk)
  TCP_SKB_CB(skb)->end_seq);
 
  tail = skb_peek_tail(&sk->sk_receive_queue);
- eaten = tail && tcp_try_coalesce(sk, RCV_QUEUE,
- tail, skb, &fragstolen);
+ eaten = tail && tcp_try_coalesce(sk, tail, skb, &fragstolen);
  tcp_rcv_nxt_update(tp, TCP_SKB_CB(skb)->end_seq);
  fin = TCP_SKB_CB(skb)->tcp_flags & TCPHDR_FIN;
  if (!eaten)
@@ -4420,10 +4407,6 @@ static void tcp_data_queue_ofo(struct sock *sk, struct sk_buff *skb)
  return;
  }
 
- /* Stash tstamp to avoid being stomped on by rbnode */
- if (TCP_SKB_CB(skb)->has_rxtstamp)
- TCP_SKB_CB(skb)->swtstamp = skb->tstamp;
-
  /* Disable header prediction. */
  tp->pred_flags = 0;
  inet_csk_schedule_ack(sk);
@@ -4451,7 +4434,7 @@ static void tcp_data_queue_ofo(struct sock *sk, struct sk_buff *skb)
  /* In the typical case, we are adding an skb to the end of the list.
  * Use of ooo_last_skb avoids the O(Log(N)) rbtree lookup.
  */
- if (tcp_try_coalesce(sk, OOO_QUEUE, tp->ooo_last_skb,
+ if (tcp_try_coalesce(sk, tp->ooo_last_skb,
  skb, &fragstolen)) {
 coalesce_done:
  tcp_grow_window(sk, skb);
@@ -4502,7 +4485,7 @@ static void tcp_data_queue_ofo(struct sock *sk, struct sk_buff *skb)
  __kfree_skb(skb1);
  goto merge_right;
  }
- } else if (tcp_try_coalesce(sk, OOO_QUEUE, skb1,
+ } else if (tcp_try_coalesce(sk, skb1,
  skb, &fragstolen)) {
  goto coalesce_done;
  }
@@ -4554,7 +4537,7 @@ static int __must_check tcp_queue_rcv(struct sock *sk, struct sk_buff *skb, int
 
  __skb_pull(skb, hdrlen);
  eaten = (tail &&
- tcp_try_coalesce(sk, RCV_QUEUE, tail,
+ tcp_try_coalesce(sk, tail,
  skb, fragstolen)) ? 1 : 0;
  tcp_rcv_nxt_update(tcp_sk(sk), TCP_SKB_CB(skb)->end_seq);
  if (!eaten) {

net/sched/sch_netem.c

@@ -146,7 +146,6 @@ struct netem_sched_data {
  */
 struct netem_skb_cb {
  psched_time_t time_to_send;
- ktime_t tstamp_save;
 };
 
 
@@ -561,7 +560,6 @@ static int netem_enqueue(struct sk_buff *skb, struct Qdisc *sch,
  }
 
  cb->time_to_send = now + delay;
- cb->tstamp_save = skb->tstamp;
  ++q->counter;
  tfifo_enqueue(skb, sch);
  } else {
@@ -629,7 +627,10 @@ static struct sk_buff *netem_dequeue(struct Qdisc *sch)
  qdisc_qstats_backlog_dec(sch, skb);
  skb->next = NULL;
  skb->prev = NULL;
- skb->tstamp = netem_skb_cb(skb)->tstamp_save;
+ /* skb->dev shares skb->rbnode area,
+ * we need to restore its value.
+ */
+ skb->dev = qdisc_dev(sch);
 
 #ifdef CONFIG_NET_CLS_ACT
  /*