On Tue, Mar 2, 2021 at 7:08 AM Jakub Kicinski <k...@kernel.org> wrote:
>
> When receiver does not accept TCP Fast Open it will only ack
> the SYN, and not the data. We detect this and immediately queue
> the data for (re)transmission in tcp_rcv_fastopen_synack().
>
> In DC networks with very low RTT and without RFS the SYN-ACK
> may arrive before NIC driver reported Tx completion on
> the original SYN. In which case skb_still_in_host_queue()
> returns true and sender will need to wait for the retransmission
> timer to fire milliseconds later.
>
> Revert back to non-fast clone skbs, this way
> skb_still_in_host_queue() won't prevent the recovery flow
> from completing.
>
> Suggested-by: Eric Dumazet <eduma...@google.com>
> Fixes: 355a901e6cf1 ("tcp: make connect() mem charging friendly")
Hmmm, not sure if this Fixes: tag makes sense.
Really, if we delay TX completions by say 10 ms, other parts of the
stack will misbehave anyway.
Also, backporting this patch up to linux-3.19 is going to be tricky.
The real issue here is that skb_still_in_host_queue() can give a false positive.
I have mixed feelings here, as you can read my answer :/
Maybe skb_still_in_host_queue() signal should not be used when a part
of the SKB has been received/acknowledged by the remote peer
(in this case the SYN part).
Alternative is that drivers unable to TX complete their skbs in a
reasonable time should call skb_orphan()
to avoid skb_unclone() penalties (and this skb_still_in_host_queue() issue)
If you really want to play and delay TX completions, maybe provide a
way to disable skb_still_in_host_queue() globally,
using a static key ?
My personal WIP/hack was something like :
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index
69a545db80d2ead47ffcf2f3819a6d066e95f35d..666f6f0a6a06fece204199e07a79e21d1faf8f92
100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -5995,7 +5995,8 @@ static bool tcp_rcv_fastopen_synack(struct sock
*sk, struct sk_buff *synack,
else
tp->fastopen_client_fail = TFO_DATA_NOT_ACKED;
skb_rbtree_walk_from(data) {
- if (__tcp_retransmit_skb(sk, data, 1))
+ /* segs = -1 to bypass
skb_still_in_host_queue() check */
+ if (__tcp_retransmit_skb(sk, data, -1))
break;
}
tcp_rearm_rto(sk);
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index
fbf140a770d8e21b936369b79abbe9857537acd8..1d1489e596976e352fe7d5ccee7a6eae55fdbcce
100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -3155,8 +3155,12 @@ int __tcp_retransmit_skb(struct sock *sk,
struct sk_buff *skb, int segs)
sk->sk_sndbuf))
return -EAGAIN;
- if (skb_still_in_host_queue(sk, skb))
- return -EBUSY;
+ if (segs > 0) {
+ if (skb_still_in_host_queue(sk, skb))
+ return -EBUSY;
+ } else {
+ segs = -segs;
+ }
if (before(TCP_SKB_CB(skb)->seq, tp->snd_una)) {
if (unlikely(before(TCP_SKB_CB(skb)->end_seq, tp->snd_una))) {
> Signed-off-by: Neil Spring <ntspr...@fb.com>
> Signed-off-by: Jakub Kicinski <k...@kernel.org>
> ---
> net/ipv4/tcp_output.c | 9 ++++++++-
> 1 file changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> index fbf140a770d8..cd9461588539 100644
> --- a/net/ipv4/tcp_output.c
> +++ b/net/ipv4/tcp_output.c
> @@ -3759,9 +3759,16 @@ static int tcp_send_syn_data(struct sock *sk, struct
> sk_buff *syn)
> /* limit to order-0 allocations */
> space = min_t(size_t, space, SKB_MAX_HEAD(MAX_TCP_HEADER));
>
> - syn_data = sk_stream_alloc_skb(sk, space, sk->sk_allocation, false);
> + syn_data = alloc_skb(MAX_TCP_HEADER + space, sk->sk_allocation);
> if (!syn_data)
> goto fallback;
> + if (!sk_wmem_schedule(sk, syn_data->truesize)) {
> + __kfree_skb(syn_data);
> + goto fallback;
> + }
> + skb_reserve(syn_data, MAX_TCP_HEADER);
> + INIT_LIST_HEAD(&syn_data->tcp_tsorted_anchor);
> +
> syn_data->ip_summed = CHECKSUM_PARTIAL;
> memcpy(syn_data->cb, syn->cb, sizeof(syn->cb));
> if (space) {
> --
> 2.26.2
>
