Re: [PATCH net-next] tcp: add tcp_add_backlog()

From: Eric Dumazet <edumazet@google.com>

When TCP operates in lossy environments (between 1 and 10 % packet
losses), many SACK blocks can be exchanged, and I noticed we could
drop them on busy senders, if these SACK blocks have to be queued
into the socket backlog.

While the main cause is the poor performance of RACK/SACK processing,
we can try to avoid these drops of valuable information that can lead to
spurious timeouts and retransmits.

Cause of the drops is the skb->truesize overestimation caused by :

- drivers allocating ~2048 (or more) bytes as a fragment to hold an
  Ethernet frame.

- various pskb_may_pull() calls bringing the headers into skb->head
  might have pulled all the frame content, but skb->truesize could
  not be lowered, as the stack has no idea of each fragment truesize.

The backlog drops are also more visible on bidirectional flows, since
their sk_rmem_alloc can be quite big.

Let's add some room for the backlog, as only the socket owner
can selectively take action to lower memory needs, like collapsing
receive queues or partial ofo pruning.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Yuchung Cheng <redacted>
Cc: Neal Cardwell <ncardwell@google.com>
---
 include/net/tcp.h   |    1 +
 net/ipv4/tcp_ipv4.c |   33 +++++++++++++++++++++++++++++----
 net/ipv6/tcp_ipv6.c |    5 +----
 3 files changed, 31 insertions(+), 8 deletions(-)

diff --git a/include/net/tcp.h b/include/net/tcp.h
index 25d64f6de69e1f639ed1531bf2d2df3f00fd76a2..5f5f09f6e019682ef29c864d2f43a8f247fcdd9a 100644