path: root/include
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authorIlpo Järvinen <ilpo.jarvinen@helsinki.fi>2008-11-24 21:20:15 -0800
committerDavid S. Miller <davem@davemloft.net>2008-11-24 21:20:15 -0800
commit832d11c5cd076abc0aa1eaf7be96c81d1a59ce41 (patch)
tree95b22ad16d1ff414cab39578ed8c927c2ce08723 /include
parentf58b22fd3c16444edc393a217a74208f1894b601 (diff)
tcp: Try to restore large SKBs while SACK processing
During SACK processing, most of the benefits of TSO are eaten by the SACK blocks that one-by-one fragment SKBs to MSS sized chunks. Then we're in problems when cleanup work for them has to be done when a large cumulative ACK comes. Try to return back to pre-split state already while more and more SACK info gets discovered by combining newly discovered SACK areas with the previous skb if that's SACKed as well. This approach has a number of benefits: 1) The processing overhead is spread more equally over the RTT 2) Write queue has less skbs to process (affect everything which has to walk in the queue past the sacked areas) 3) Write queue is consistent whole the time, so no other parts of TCP has to be aware of this (this was not the case with some other approach that was, well, quite intrusive all around). 4) Clean_rtx_queue can release most of the pages using single put_page instead of previous PAGE_SIZE/mss+1 calls In case a hole is fully filled by the new SACK block, we attempt to combine the next skb too which allows construction of skbs that are even larger than what tso split them to and it handles hole per on every nth patterns that often occur during slow start overshoot pretty nicely. Though this to be really useful also a retransmission would have to get lost since cumulative ACKs advance one hole at a time in the most typical case. TODO: handle upwards only merging. That should be rather easy when segment is fully sacked but I'm leaving that as future work item (it won't make very large difference anyway since this current approach already covers quite a lot of normal cases). I was earlier thinking of some sophisticated way of tracking timestamps of the first and the last segment but later on realized that it won't be that necessary at all to store the timestamp of the last segment. The cases that can occur are basically either: 1) ambiguous => no sensible measurement can be taken anyway 2) non-ambiguous is due to reordering => having the timestamp of the last segment there is just skewing things more off than does some good since the ack got triggered by one of the holes (besides some substle issues that would make determining right hole/skb even harder problem). Anyway, it has nothing to do with this change then. I choose to route some abnormal looking cases with goto noop, some could be handled differently (eg., by stopping the walking at that skb but again). In general, they either shouldn't happen at all or are rare enough to make no difference in practice. In theory this change (as whole) could cause some macroscale regression (global) because of cache misses that are taken over the round-trip time but it gets very likely better because of much less (local) cache misses per other write queue walkers and the big recovery clearing cumulative ack. Worth to note that these benefits would be very easy to get also without TSO/GSO being on as long as the data is in pages so that we can merge them. Currently I won't let that happen because DSACK splitting at fragment that would mess up pcounts due to sk_can_gso in tcp_set_skb_tso_segs. Once DSACKs fragments gets avoided, we have some conditions that can be made less strict. TODO: I will probably have to convert the excessive pointer passing to struct sacktag_state... :-) My testing revealed that considerable amount of skbs couldn't be shifted because they were cloned (most likely still awaiting tx reclaim)... [The rest is considering future work instead since I got repeatably EFAULT to tcpdump's recvfrom when I added pskb_expand_head to deal with clones, so I separated that into another, later patch] ...To counter that, I gave up on the fifth advantage: 5) When growing previous SACK block, less allocs for new skbs are done, basically a new alloc is needed only when new hole is detected and when the previous skb runs out of frags space ...which now only happens of if reclaim is fast enough to dispose the clone before the SACK block comes in (the window is RTT long), otherwise we'll have to alloc some. With clones being handled I got these numbers (will be somewhat worse without that), taken with fine-grained mibs: TCPSackShifted 398 TCPSackMerged 877 TCPSackShiftFallback 320 TCPSACKCOLLAPSEFALLBACKGSO 0 TCPSACKCOLLAPSEFALLBACKSKBBITS 0 TCPSACKCOLLAPSEFALLBACKSKBDATA 0 TCPSACKCOLLAPSEFALLBACKBELOW 0 TCPSACKCOLLAPSEFALLBACKFIRST 1 TCPSACKCOLLAPSEFALLBACKPREVBITS 318 TCPSACKCOLLAPSEFALLBACKMSS 1 TCPSACKCOLLAPSEFALLBACKNOHEAD 0 TCPSACKCOLLAPSEFALLBACKSHIFT 0 TCPSACKCOLLAPSENOOPSEQ 0 TCPSACKCOLLAPSENOOPSMALLPCOUNT 0 TCPSACKCOLLAPSENOOPSMALLLEN 0 TCPSACKCOLLAPSEHOLE 12 Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi> Signed-off-by: David S. Miller <davem@davemloft.net>
Diffstat (limited to 'include')
2 files changed, 38 insertions, 0 deletions
diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index a01b6f84e3b..acf17af45af 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -493,6 +493,19 @@ static inline bool skb_queue_is_last(const struct sk_buff_head *list,
+ * skb_queue_is_first - check if skb is the first entry in the queue
+ * @list: queue head
+ * @skb: buffer
+ *
+ * Returns true if @skb is the first buffer on the list.
+ */
+static inline bool skb_queue_is_first(const struct sk_buff_head *list,
+ const struct sk_buff *skb)
+ return (skb->prev == (struct sk_buff *) list);
* skb_queue_next - return the next packet in the queue
* @list: queue head
* @skb: current buffer
@@ -511,6 +524,24 @@ static inline struct sk_buff *skb_queue_next(const struct sk_buff_head *list,
+ * skb_queue_prev - return the prev packet in the queue
+ * @list: queue head
+ * @skb: current buffer
+ *
+ * Return the prev packet in @list before @skb. It is only valid to
+ * call this if skb_queue_is_first() evaluates to false.
+ */
+static inline struct sk_buff *skb_queue_prev(const struct sk_buff_head *list,
+ const struct sk_buff *skb)
+ /* This BUG_ON may seem severe, but if we just return then we
+ * are going to dereference garbage.
+ */
+ BUG_ON(skb_queue_is_first(list, skb));
+ return skb->prev;
* skb_get - reference buffer
* @skb: buffer to reference
@@ -1652,6 +1683,8 @@ extern int skb_splice_bits(struct sk_buff *skb,
extern void skb_copy_and_csum_dev(const struct sk_buff *skb, u8 *to);
extern void skb_split(struct sk_buff *skb,
struct sk_buff *skb1, const u32 len);
+extern int skb_shift(struct sk_buff *tgt, struct sk_buff *skb,
+ int shiftlen);
extern struct sk_buff *skb_segment(struct sk_buff *skb, int features);
diff --git a/include/net/tcp.h b/include/net/tcp.h
index 90b4c3b4c33..265392470b2 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -1192,6 +1192,11 @@ static inline struct sk_buff *tcp_write_queue_next(struct sock *sk, struct sk_bu
return skb_queue_next(&sk->sk_write_queue, skb);
+static inline struct sk_buff *tcp_write_queue_prev(struct sock *sk, struct sk_buff *skb)
+ return skb_queue_prev(&sk->sk_write_queue, skb);
#define tcp_for_write_queue(skb, sk) \
skb_queue_walk(&(sk)->sk_write_queue, skb)