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

[Jon Maloy <jmaloy@redhat.com>]

Hi David,
This is the last comment I received from you regarding this patch.
See below for further comment.

On 2024-05-16 00:16, David Gibson wrote:
> On Wed, May 15, 2024 at 10:57:06PM -0400, Jon Maloy wrote:
>>
>> On 2024-05-15 22:24, David Gibson wrote:
>>> On Wed, May 15, 2024 at 11:34:27AM -0400, Jon Maloy 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
>>> You can make the 2d array explicit in the type as:
>>> 	struct iovec (*frames)[TCP_NUM_IOVS];
>>> See, for example the 'tap_iov' local in udp_tap_send().   (I recommend
>>> the command line tool 'cdecl', also available online at cdecl.org for
>>> working out confusing pointer-to-array types).
>> Nice. I  wasn't quite happy with this.
>>>> + * @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) {
>>> Nit: I find having the two parallel counters kind of confusing.  It
>>> naturally goes away with the type change suggested above, but even
>>> without that I'd prefer an explicit multiply in the body.  I strongly
>>> suspect the compiler will be better at working out if the strength
>>> reduction is worth it.
>>>
>>>> +		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))
>>> Isn't this test inverted?  We want to rewind seq_to_tap if seq is less
>>> than it, rather than the other way aruond.
>> No. We do 'continue', i.e., nothing, if this condition is fulfilled.
>> This may look a little non-intuitive here, but makes sense when I add the
>> next patch.
> Oh, of course, my mistake.
>

The code now (v7) looks as follows:

/**
  * 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)[TCP_NUM_IOVS],
                            int num_frames)
{
         int i;

         for (i = 0; i < num_frames; i++) {
                 struct tcp_tap_conn *conn = conns[i];
                 struct tcphdr *th = frames[i][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_set_peek_offset(conn->sock, seq - 
conn->seq_ack_from_tap);
         }
}

/**
  * tcp_payload_flush() - Send out buffers for segments with data
  * @c:          Execution context
  */
static void tcp_payload_flush(const struct ctx *c)
{
         size_t m;

         m = tap_send_frames(c, &tcp6_l2_iov[0][0], TCP_NUM_IOVS,
                             tcp6_payload_used);
         if (m != tcp6_payload_used) {
                 tcp_revert_seq(tcp6_frame_conns, &tcp6_l2_iov[m],
                                tcp6_payload_used - m);
         }
         tcp6_payload_used = 0;

         m = tap_send_frames(c, &tcp4_l2_iov[0][0], TCP_NUM_IOVS,
                             tcp4_payload_used);
         if (m != tcp4_payload_used) {
                 tcp_revert_seq(tcp4_frame_conns, &tcp4_l2_iov[m],
                                tcp4_payload_used - m);
         }
         tcp4_payload_used = 0;
}

Was this the version you were talking about on Monday morning?
Did you spot some bug here which I am missing?

Thanks
///jon