Re: [PATCH v4 1/3] tcp: move seq_to_tap update to when frame is queued

[Stefano Brivio <sbrivio@redhat.com>]

On Wed, 15 May 2024 11:34:27 -0400
Jon Maloy <jmaloy@redhat.com> wrote:

> commit a469fc393fa1 ("tcp, tap: Don't increase tap-side sequence counter for dropped frames")
> delayed update of conn->seq_to_tap until the moment the corresponding
> frame has been successfully pushed out. This has the advantage that we
> immediately can make a new attempt to transmit a frame after a failed
> trasnmit, rather than waiting for the peer to later discover a gap and
> trigger the fast retransmit mechanism to solve the problem.
> 
> This approach has turned out to cause a problem with spurious sequence
> number updates during peer-initiated retransmits, and we have realized
> it may not be the best way to solve the above issue.
> 
> We now restore the previous method, by updating the said field at the
> moment a frame is added to the outqueue. To retain the advantage of
> having a quick re-attempt based on local failure detection, we now scan
> through the part of the outqueue that had do be dropped, and restore the
> sequence counter for each affected connection to the most appropriate
> value.
> 
> Signed-off-by: Jon Maloy <jmaloy@redhat.com>
> 
> ---
> v2: - Re-spun loop in tcp_revert_seq() and some other changes based on
>       feedback from Stefano Brivio.
>     - Added paranoid test to avoid that seq_to_tap becomes lower than
>       seq_ack_from_tap.
> 
> v3: - Identical to v2. Called v3 because it was embedded in a series
>       with that version.
> 
> v4: - In tcp_revert_seq(), we read the sequence number from the TCP
>       header instead of keeping a copy in struct tcp_buf_seq_update.
>     - Since the only remaining field in struct tcp_buf_seq_update is
>       a pointer to struct tcp_tap_conn, we eliminate the struct
>       altogether, and make the tcp6/tcp3_buf_seq_update arrays into
>       arrays of said pointer.
>     - Removed 'paranoid' test in tcp_revert_seq. If it happens, it
>       is not fatal, and will be caught by other code anyway.
>     - Separated from the series again.
> ---
>  tcp.c | 59 +++++++++++++++++++++++++++++++++++++----------------------
>  1 file changed, 37 insertions(+), 22 deletions(-)
> 
> diff --git a/tcp.c b/tcp.c
> index 21d0af0..976dba8 100644
> --- a/tcp.c
> +++ b/tcp.c
> @@ -410,16 +410,6 @@ static int tcp_sock_ns		[NUM_PORTS][IP_VERSIONS];
>   */
>  static union inany_addr low_rtt_dst[LOW_RTT_TABLE_SIZE];
>  
> -/**
> - * tcp_buf_seq_update - Sequences to update with length of frames once sent
> - * @seq:	Pointer to sequence number sent to tap-side, to be updated
> - * @len:	TCP payload length
> - */
> -struct tcp_buf_seq_update {
> -	uint32_t *seq;
> -	uint16_t len;
> -};
> -
>  /* Static buffers */
>  /**
>   * struct tcp_payload_t - TCP header and data to send segments with payload
> @@ -461,7 +451,8 @@ static struct tcp_payload_t	tcp4_payload[TCP_FRAMES_MEM];
>  
>  static_assert(MSS4 <= sizeof(tcp4_payload[0].data), "MSS4 is greater than 65516");
>  
> -static struct tcp_buf_seq_update tcp4_seq_update[TCP_FRAMES_MEM];
> +/* References tracking the owner connection of frames in the tap outqueue */
> +static struct tcp_tap_conn *tcp4_frame_conns[TCP_FRAMES_MEM];
>  static unsigned int tcp4_payload_used;
>  
>  static struct tap_hdr		tcp4_flags_tap_hdr[TCP_FRAMES_MEM];
> @@ -483,7 +474,8 @@ static struct tcp_payload_t	tcp6_payload[TCP_FRAMES_MEM];
>  
>  static_assert(MSS6 <= sizeof(tcp6_payload[0].data), "MSS6 is greater than 65516");
>  
> -static struct tcp_buf_seq_update tcp6_seq_update[TCP_FRAMES_MEM];
> +/* References tracking the owner connection of frames in the tap outqueue */
> +static struct tcp_tap_conn *tcp6_frame_conns[TCP_FRAMES_MEM];
>  static unsigned int tcp6_payload_used;
>  
>  static struct tap_hdr		tcp6_flags_tap_hdr[TCP_FRAMES_MEM];
> @@ -1261,25 +1253,49 @@ static void tcp_flags_flush(const struct ctx *c)
>  	tcp4_flags_used = 0;
>  }
>  
> +/**
> + * tcp_revert_seq() - Revert affected conn->seq_to_tap after failed transmission
> + * @conns:       Array of connection pointers corresponding to queued frames
> + * @frames:      Two-dimensional array containing queued frames with sub-iovs
> + * @num_frames:  Number of entries in the two arrays to be compared
> + */
> +static void tcp_revert_seq(struct tcp_tap_conn **conns, struct iovec *frames,
> +			   int num_frames)
> +{
> +	int c, f;
> +
> +	for (c = 0, f = 0; c < num_frames; c++, f += TCP_NUM_IOVS) {
> +		struct tcp_tap_conn *conn = conns[c];
> +		struct tcphdr *th = frames[f + TCP_IOV_PAYLOAD].iov_base;
> +		uint32_t seq = ntohl(th->seq);
> +
> +		if (SEQ_LE(conn->seq_to_tap, seq))
> +			continue;
> +
> +		conn->seq_to_tap = seq;
> +	}
> +}
> +
>  /**
>   * tcp_payload_flush() - Send out buffers for segments with data
>   * @c:		Execution context
>   */
>  static void tcp_payload_flush(const struct ctx *c)
>  {
> -	unsigned i;
>  	size_t m;
>  
>  	m = tap_send_frames(c, &tcp6_l2_iov[0][0], TCP_NUM_IOVS,
>  			    tcp6_payload_used);
> -	for (i = 0; i < m; i++)
> -		*tcp6_seq_update[i].seq += tcp6_seq_update[i].len;
> +	if (m != tcp6_payload_used)
> +		tcp_revert_seq(tcp6_frame_conns, &tcp6_l2_iov[m][0],
> +			       tcp6_payload_used - m);

Nit, not worth respinning, and I can fix this up on merge: we always
use curly brackets around multiple lines, even if it's a single
statement, consistently with the current Linux kernel coding style.

>  	tcp6_payload_used = 0;
>  
>  	m = tap_send_frames(c, &tcp4_l2_iov[0][0], TCP_NUM_IOVS,
>  			    tcp4_payload_used);
> -	for (i = 0; i < m; i++)
> -		*tcp4_seq_update[i].seq += tcp4_seq_update[i].len;
> +	if (m != tcp4_payload_used)
> +		tcp_revert_seq(tcp4_frame_conns, &tcp4_l2_iov[m][0],
> +			       tcp4_payload_used - m);

Same here.

-- 
Stefano