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UPSTREAM: tcp: fix DSACK undo in fast recovery to call tcp_try_to_open() 

JIRA: https://issues.redhat.com/browse/RHEL-62865
Tested: LNST, Tier1

Upstream commit:
commit a6458ab7fd4f427d4f6f54380453ad255b7fde83
Author: Neal Cardwell <ncardwell@google.com>
Date:   Wed Jun 26 22:42:27 2024 -0400

    UPSTREAM: tcp: fix DSACK undo in fast recovery to call tcp_try_to_open()

    In some production workloads we noticed that connections could
    sometimes close extremely prematurely with ETIMEDOUT after
    transmitting only 1 TLP and RTO retransmission (when we would normally
    expect roughly tcp_retries2 = TCP_RETR2 = 15 RTOs before a connection
    closes with ETIMEDOUT).

    From tracing we determined that these workloads can suffer from a
    scenario where in fast recovery, after some retransmits, a DSACK undo
    can happen at a point where the scoreboard is totally clear (we have
    retrans_out == sacked_out == lost_out == 0). In such cases, calling
    tcp_try_keep_open() means that we do not execute any code path that
    clears tp->retrans_stamp to 0. That means that tp->retrans_stamp can
    remain erroneously set to the start time of the undone fast recovery,
    even after the fast recovery is undone. If minutes or hours elapse,
    and then a TLP/RTO/RTO sequence occurs, then the start_ts value in
    retransmits_timed_out() (which is from tp->retrans_stamp) will be
    erroneously ancient (left over from the fast recovery undone via
    DSACKs). Thus this ancient tp->retrans_stamp value can cause the
    connection to die very prematurely with ETIMEDOUT via
    tcp_write_err().

    The fix: we change DSACK undo in fast recovery (TCP_CA_Recovery) to
    call tcp_try_to_open() instead of tcp_try_keep_open(). This ensures
    that if no retransmits are in flight at the time of DSACK undo in fast
    recovery then we properly zero retrans_stamp. Note that calling
    tcp_try_to_open() is more consistent with other loss recovery
    behavior, since normal fast recovery (CA_Recovery) and RTO recovery
    (CA_Loss) both normally end when tp->snd_una meets or exceeds
    tp->high_seq and then in tcp_fastretrans_alert() the "default" switch
    case executes tcp_try_to_open(). Also note that by inspection this
    change to call tcp_try_to_open() implies at least one other nice bug
    fix, where now an ECE-marked DSACK that causes an undo will properly
    invoke tcp_enter_cwr() rather than ignoring the ECE mark.

    Fixes: c7d9d6a185 ("tcp: undo on DSACK during recovery")
    Signed-off-by: Neal Cardwell <ncardwell@google.com>
    Signed-off-by: Yuchung Cheng <ycheng@google.com>
    Signed-off-by: Eric Dumazet <edumazet@google.com>
    Signed-off-by: David S. Miller <davem@davemloft.net>

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
This commit is contained in:
Paolo Abeni 2024-10-16 15:49:10 +02:00
parent 37a7b087d1
commit 212db5de55
1 changed files with 1 additions and 1 deletions
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2
net/ipv4/tcp_input.c
View File

@ -3014,7 +3014,7 @@ static void tcp_fastretrans_alert(struct sock *sk, const u32 prior_snd_una,
return;
if (tcp_try_undo_dsack(sk))
tcp_try_keep_open(sk);
tcp_try_to_open(sk, flag);
tcp_identify_packet_loss(sk, ack_flag);
if (icsk->icsk_ca_state != TCP_CA_Recovery) {