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tcp: Reset ACK_FROM_TAP_DUE flag only as needed, update timer

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David reports that TCP transfers might stall, especially with smaller
socket buffer sizes, because we reset the ACK_FROM_TAP_DUE flag, in
tcp_tap_handler(), whenever we receive an ACK segment, regardless of
its sequence number and the fact that we might still be waiting for
one. This way, we might fail to re-transmit frames on ACK timeouts.

We need, instead, to:

- indicate with the @retrans field only re-transmissions for the same
  data sequences. If we make progress, it should be reset, given that
  it's used to abort a connection when we exceed a given number of
  re-transmissions for the same data

- unset the ACK_FROM_TAP_DUE flag if and only if the acknowledged
  sequence is the same as the last one we sent, as suggested by David

- keep it set otherwise, if progress was done but not all the data we
  sent was acknowledged, and update the expiration of the ACK timeout

Add a new helper for these purposes, tcp_update_seqack_from_tap().

To extend the ACK timeout, the new helper sets the ACK_FROM_TAP_DUE
flag, even if it was already set, and conn_flag_do() triggers a timer
update. This part should be revisited at a later time, because,
strictly speaking, ACK_FROM_TAP_DUE isn't a flag anymore. One
possibility might be to introduce another connection attribute for
events affecting timer deadlines.

Reported-by: David Gibson <david@gibson.dropbear.id.au>
Link: https://bugs.passt.top/show_bug.cgi?id=41
Suggested-by: David Gibson <david@gibson.dropbear.id.au>
Fixes: be5bbb9b06 ("tcp: Rework timers to use timerfd instead of periodic bitmap scan")
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
This commit is contained in:
Stefano Brivio 2023-02-12 22:26:55 +01:00
parent ac153595c0
commit cc6d8286d1
1 changed files with 38 additions and 14 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

52
tcp.c
View file

@ -757,8 +757,18 @@ static void conn_flag_do(const struct ctx *c, struct tcp_tap_conn *conn,
tcp_flag_str[fls(~flag)]);
}
} else {
if (conn->flags & flag)
if (conn->flags & flag) {
/* Special case: setting ACK_FROM_TAP_DUE on a
* connection where it's already set is used to
* re-schedule the existing timer.
* TODO: define clearer semantics for timer-related
* flags and factor this into the logic below.
*/
if (flag == ACK_FROM_TAP_DUE)
tcp_timer_ctl(c, conn);
return;
}
conn->flags |= flag;
if (fls(flag) >= 0) {
@ -1591,6 +1601,26 @@ out:
conn->seq_ack_to_tap != prev_ack_to_tap;
}
/**
* tcp_update_seqack_from_tap() - ACK number from tap and related flags/counters
* @c: Execution context
* @conn: Connection pointer
* @seq Current ACK sequence, host order
*/
static void tcp_update_seqack_from_tap(const struct ctx *c,
struct tcp_tap_conn *conn, uint32_t seq)
{
if (SEQ_GT(seq, conn->seq_ack_from_tap)) {
if (seq == conn->seq_to_tap)
conn_flag(c, conn, ~ACK_FROM_TAP_DUE);
else
conn_flag(c, conn, ACK_FROM_TAP_DUE);
conn->retrans = 0;
conn->seq_ack_from_tap = seq;
}
}
/**
* tcp_send_flag() - Send segment with flags to tap (no payload)
* @c: Execution context
@ -2041,7 +2071,6 @@ static int tcp_sock_consume(struct tcp_tap_conn *conn, uint32_t ack_seq)
MSG_DONTWAIT | MSG_TRUNC) < 0)
return -errno;
conn->seq_ack_from_tap = ack_seq;
return 0;
}
@ -2333,14 +2362,9 @@ static void tcp_data_from_tap(struct ctx *c, struct tcp_tap_conn *conn,
tcp_clamp_window(c, conn, max_ack_seq_wnd);
if (ack) {
if (max_ack_seq == conn->seq_to_tap) {
conn_flag(c, conn, ~ACK_FROM_TAP_DUE);
conn->retrans = 0;
}
tcp_sock_consume(conn, max_ack_seq);
}
/* On socket flush failure, pretend there was no ACK, try again later */
if (ack && !tcp_sock_consume(conn, max_ack_seq))
tcp_update_seqack_from_tap(c, conn, max_ack_seq);
if (retr) {
trace("TCP: fast re-transmit, ACK: %u, previous sequence: %u",
@ -2492,10 +2516,8 @@ int tcp_tap_handler(struct ctx *c, int af, const void *addr,
return p->count;
}
if (th->ack) {
conn_flag(c, conn, ~ACK_FROM_TAP_DUE);
conn->retrans = 0;
}
if (th->ack && !(conn->events & ESTABLISHED))
tcp_update_seqack_from_tap(c, conn, ntohl(th->ack_seq));
conn_flag(c, conn, ~STALLED);
@ -2543,6 +2565,8 @@ int tcp_tap_handler(struct ctx *c, int af, const void *addr,
/* Established connections not accepting data from tap */
if (conn->events & TAP_FIN_RCVD) {
tcp_update_seqack_from_tap(c, conn, ntohl(th->ack_seq));
if (conn->events & SOCK_FIN_RCVD &&
conn->seq_ack_from_tap == conn->seq_to_tap)
conn_event(c, conn, CLOSED);