Re: [PATCH] tcp: move seq_to_tap update to when frame is queued

[David Gibson <david@gibson.dropbear.id.au>]

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On Fri, May 10, 2024 at 06:40:30PM +0200, Stefano Brivio wrote:
> On Wed,  8 May 2024 23:00:23 -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 retransmit a buffer that we fail to trasnmit, rather
> > than waiting for the peer side to discover the loss and initiate fast
> > retransmit.
> 
> It's not really fast retransmit, it's a simple retry of the operation
> that didn't succeed. We didn't even transmit.
> 
> > 
> > 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 te 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 fast
> > retansmit
> 
> Same here.
> 
> > 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>
> > ---
> >  tcp.c | 52 ++++++++++++++++++++++++++++++++++++++++++----------
> >  1 file changed, 42 insertions(+), 10 deletions(-)
> > 
> > diff --git a/tcp.c b/tcp.c
> > index 21d0af0..58fdbc9 100644
> > --- a/tcp.c
> > +++ b/tcp.c
> > @@ -412,11 +412,13 @@ static union inany_addr low_rtt_dst[LOW_RTT_TABLE_SIZE];
> >  
> >  /**
> >   * tcp_buf_seq_update - Sequences to update with length of frames once sent
> 
> This is not the case anymore, maybe:
> 
>  * tcp_conn_old_seq() - Old sequence numbers for connections with pending frames
> 
> > - * @seq:	Pointer to sequence number sent to tap-side, to be updated
> > + * @conn:       Pointer to connection corresponding to frame. May need update
> 
> Mixed whitespace and tabs. It looks like the connection pointer might
> need to be updated... what about:
> 
>  * @conn:	Pointer to connection for this frame
> 
> ?
> 
> > + * @seq:	Sequence number of the corresponding frame
> >   * @len:	TCP payload length
> 
> The length is not needed anymore.

Strictly speaking, I don't think you need the sequence number here
either: it should be in the frame itself.  The fiddliness of
extracting it from the buffer might make it worthwhile to store here
anyway.

> >   */
> >  struct tcp_buf_seq_update {
> > -	uint32_t *seq;
> > +	struct tcp_tap_conn *conn;
> > +	uint32_t seq;
> >  	uint16_t len;
> >  };
> >  
> > @@ -1261,25 +1263,52 @@ 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
> > + * @seq_update: Array with connection and sequence number data
> > + * @s:          Entry corresponding to first dropped frame
> > + * @e:          Entry corresponding to last dropped frame
> 
> These are not pointer to the entries, though. They are indices of the
> queued frames.
> 
> > + */
> > +static void tcp_revert_seq(struct tcp_buf_seq_update *seq_update, int s, int e)
> > +{
> > +	struct tcp_tap_conn *conn;
> > +	uint32_t lowest_seq;
> > +	int i, ii;
> > +
> > +	for (i = s; i < e; i++) {
> > +		conn = seq_update[i].conn;
> > +		lowest_seq = seq_update[i].seq;
> > +
> > +		for (ii = i + 1; ii < e; ii++) {
> > +			if (seq_update[ii].conn != conn)
> > +				continue;
> > +			if (SEQ_GT(lowest_seq, seq_update[ii].seq))
> > +				lowest_seq = seq_update[ii].seq;
> > +		}
> 
> If I recall correctly, David suggested a simpler approach that avoids
> this O(n^2) scan, based on the observation that 1. the first entry you
> find in the table also has the lowest sequence number (we don't send
> frames out-of-order), and that 2. you'll never revert to a higher
> sequence number (the two lines below take care of that).

Right..

> That is, you could just scan the table once, and if you find a sequence
> number that's lower than the current sequence stored for the connection,
> store it.
> 
> > +
> > +		if (SEQ_GT(conn->seq_to_tap, lowest_seq))
> > +			conn->seq_to_tap = lowest_seq;

..these lines here, specifically.  Basically we rewind seq_to_tap each
time we find an untransmitted frame that sits before it.
Theoretically that could involve multiple rewinds, but a) that's not
fatal, merely suboptimal and b) it won't happen in practice, since
frames in the queue will (nearly?) always have increasing sequence
numbers.

> > +	}
> > +}
> > +
> >  /**
> >   * 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_seq_update, m, tcp6_payload_used);
> >  	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_seq_update, m, tcp4_payload_used);
> >  	tcp4_payload_used = 0;
> >  }
> >  
> > @@ -2129,10 +2158,11 @@ static int tcp_sock_consume(const struct tcp_tap_conn *conn, uint32_t ack_seq)
> >  static void tcp_data_to_tap(const struct ctx *c, struct tcp_tap_conn *conn,
> >  			    ssize_t dlen, int no_csum, uint32_t seq)
> >  {
> > -	uint32_t *seq_update = &conn->seq_to_tap;
> >  	struct iovec *iov;
> >  	size_t l4len;
> >  
> > +	conn->seq_to_tap = seq;
> 
> This is the sequence number for the frame we're sending (start of this
> frame), but not the current byte sequence sent to the "tap" (end of
> this frame), which would be seq + dlen, I think.
> 
> > +
> >  	if (CONN_V4(conn)) {
> >  		struct iovec *iov_prev = tcp4_l2_iov[tcp4_payload_used - 1];
> >  		const uint16_t *check = NULL;
> > @@ -2142,7 +2172,8 @@ static void tcp_data_to_tap(const struct ctx *c, struct tcp_tap_conn *conn,
> >  			check = &iph->check;
> >  		}
> >  
> > -		tcp4_seq_update[tcp4_payload_used].seq = seq_update;
> > +		tcp4_seq_update[tcp4_payload_used].conn = conn;
> > +		tcp4_seq_update[tcp4_payload_used].seq = seq;
> >  		tcp4_seq_update[tcp4_payload_used].len = dlen;
> >  
> >  		iov = tcp4_l2_iov[tcp4_payload_used++];
> > @@ -2151,7 +2182,8 @@ static void tcp_data_to_tap(const struct ctx *c, struct tcp_tap_conn *conn,
> >  		if (tcp4_payload_used > TCP_FRAMES_MEM - 1)
> >  			tcp_payload_flush(c);
> >  	} else if (CONN_V6(conn)) {
> > -		tcp6_seq_update[tcp6_payload_used].seq = seq_update;
> > +		tcp6_seq_update[tcp6_payload_used].conn = conn;
> > +		tcp6_seq_update[tcp6_payload_used].seq = seq;
> >  		tcp6_seq_update[tcp6_payload_used].len = dlen;
> >  
> >  		iov = tcp6_l2_iov[tcp6_payload_used++];
> 

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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