Re: [PATCH v3 2/3] tcp: leverage support of SO_PEEK_OFF socket option when available

[Jon Maloy <jmaloy@redhat.com>]

On 2024-05-14 13:22, Stefano Brivio wrote:
> On Sat, 11 May 2024 11:20:07 -0400
> Jon Maloy <jmaloy@redhat.com> wrote:
>
>>  From linux-6.9.0 the kernel will contain
>> commit 05ea491641d3 ("tcp: add support for SO_PEEK_OFF socket option").
>>
>> This new feature makes is possible to call recv_msg(MSG_PEEK) and make
>> it start reading data from a given offset set by the SO_PEEK_OFF socket
>> option. This way, we can avoid repeated reading of already read bytes of
>> a received message, hence saving read cycles when forwarding TCP
>> messages in the host->name space direction.
>>
>> In this commit, we add functionality to leverage this feature when
>> available, while we fall back to the previous behavior when not.
>>
>> Measurements with iperf3 shows that throughput increases with 15-20
>> percent in the host->namespace direction when this feature is used.
>>
>> Signed-off-by: Jon Maloy <jmaloy@redhat.com>
>>
>> ---
>> v2: - Some smaller changes as suggested by David Gibson and Stefano Brivio.
>>      - Moved initial set_peek_offset(0) to only the locations where the socket is set
>>        to ESTABLISHED.
>>      - Removed the per-packet synchronization between sk_peek_off and
>>        already_sent. Instead only doing it in retransmit situations.
>>      - The problem I found when trouble shooting the occasionally occurring
>>        out of synch values between 'already_sent' and 'sk_peek_offset' may
>>        have deeper implications that we may need to be investigate.
>>
>> v3: - Rebased to most recent version of tcp.c, plus the previous
>>        patch in this series.
>>      - Some changes based on feedback from PASST team
>> ---
>>   tcp.c | 77 ++++++++++++++++++++++++++++++++++++++++++++++++-----------
>>   1 file changed, 63 insertions(+), 14 deletions(-)
>>
>> diff --git a/tcp.c b/tcp.c
>> index 21cbfba..8297812 100644
>> --- a/tcp.c
>> +++ b/tcp.c
>> @@ -520,6 +520,9 @@ static struct iovec	tcp6_l2_iov		[TCP_FRAMES_MEM][TCP_NUM_IOVS];
>>   static struct iovec	tcp4_l2_flags_iov	[TCP_FRAMES_MEM][TCP_NUM_IOVS];
>>   static struct iovec	tcp6_l2_flags_iov	[TCP_FRAMES_MEM][TCP_NUM_IOVS];
>>   
>> +/* Does the kernel support TCP_PEEK_OFF? */
>> +static bool peek_offset_cap;
>> +
>>   /* sendmsg() to socket */
>>   static struct iovec	tcp_iov			[UIO_MAXIOV];
>>   
>> @@ -535,6 +538,20 @@ static_assert(ARRAY_SIZE(tc_hash) >= FLOW_MAX,
>>   int init_sock_pool4		[TCP_SOCK_POOL_SIZE];
>>   int init_sock_pool6		[TCP_SOCK_POOL_SIZE];
>>   
>> +/**
>> + * tcp_set_peek_offset() - Set SO_PEEK_OFF offset on a socket if supported
>> + * @s:          Socket to update
>> + * @offset:     Offset in bytes
>> + */
>> +static void tcp_set_peek_offset(int s, int offset)
>> +{
>> +	if (!peek_offset_cap)
>> +		return;
>> +
>> +	if (setsockopt(s, SOL_SOCKET, SO_PEEK_OFF, &offset, sizeof(offset)))
>> +		err("Failed to set SO_PEEK_OFF");
> It would be nice to say on which socket and at which offset, in case it
> failed.
ok.
>
>> +}
>> +
>>   /**
>>    * tcp_conn_epoll_events() - epoll events mask for given connection state
>>    * @events:	Current connection events
>> @@ -1280,11 +1297,14 @@ static void tcp_revert_seq(struct tcp_frame_ref *frame_ref, int first, int last)
>>   			continue;
>>   
>>   		conn->seq_to_tap = frame_ref[i].seq;
>> +		tcp_set_peek_offset(conn->sock,
>> +				    conn->seq_to_tap - conn->seq_ack_from_tap);
>>   
>>   		if (SEQ_GE(conn->seq_to_tap, conn->seq_ack_from_tap))
>>   			continue;
>>   
>>   		conn->seq_to_tap = conn->seq_ack_from_tap;
>> +		tcp_set_peek_offset(conn->sock, 0);
>>   	}
>>   }
>>   
>> @@ -2203,42 +2223,52 @@ static int tcp_data_from_sock(struct ctx *c, struct tcp_tap_conn *conn)
>>   	uint32_t wnd_scaled = conn->wnd_from_tap << conn->ws_from_tap;
>>   	int fill_bufs, send_bufs = 0, last_len, iov_rem = 0;
>>   	int sendlen, len, dlen, v4 = CONN_V4(conn);
>> +	uint32_t max_send, seq, already_sent;
>>   	int s = conn->sock, i, ret = 0;
>>   	struct msghdr mh_sock = { 0 };
>>   	uint16_t mss = MSS_GET(conn);
>> -	uint32_t already_sent, seq;
>>   	struct iovec *iov;
>>   
>> +	/* How much have we read/sent since last received ack ? */
>>   	already_sent = conn->seq_to_tap - conn->seq_ack_from_tap;
>> -
> Spurious change.
No. Intentional.
>
>>   	if (SEQ_LT(already_sent, 0)) {
>>   		/* RFC 761, section 2.1. */
>>   		flow_trace(conn, "ACK sequence gap: ACK for %u, sent: %u",
>>   			   conn->seq_ack_from_tap, conn->seq_to_tap);
>>   		conn->seq_to_tap = conn->seq_ack_from_tap;
>>   		already_sent = 0;
>> +		tcp_set_peek_offset(s, 0);
>>   	}
>>   
>> -	if (!wnd_scaled || already_sent >= wnd_scaled) {
>> +	/* How much are we still allowed to send within current window ? */
>> +	max_send = conn->seq_ack_from_tap + wnd_scaled - conn->seq_to_tap;
> I'm not sure about the purpose of this whole part of the patch, even if
> I try to see it in the context of 3/3.
To me it simply makes the code more readable, at least after the next 
patch.
Also, we you pointed out that we shouldn't really be dealing with windows
in this function, and that will be eliminated altogether in the next patch.
I can move it back to the next patch, but the code will look the same.
>
>> +	if (SEQ_LE(max_send, 0)) {
>> +		flow_trace(conn, "Empty window: win: %u, sent: %u",
>> +			   wnd_scaled, conn->seq_to_tap);
>>   		conn_flag(c, conn, STALLED);
>>   		conn_flag(c, conn, ACK_FROM_TAP_DUE);
>>   		return 0;
>>   	}
>>   
>> -	/* Set up buffer descriptors we'll fill completely and partially. */
>> -	fill_bufs = DIV_ROUND_UP(wnd_scaled - already_sent, mss);
>> +	/* Set up buffer descriptors to fill completely or partially. */
> Spurious change... or did you really mean to change this comment?
Yes.
>
>> +	fill_bufs = DIV_ROUND_UP(max_send, mss);
>>   	if (fill_bufs > TCP_FRAMES) {
>>   		fill_bufs = TCP_FRAMES;
>>   		iov_rem = 0;
>>   	} else {
>> -		iov_rem = (wnd_scaled - already_sent) % mss;
>> +		iov_rem = max_send % mss;
>>   	}
>>   
>> -	mh_sock.msg_iov = iov_sock;
>> -	mh_sock.msg_iovlen = fill_bufs + 1;
>> -
>> -	iov_sock[0].iov_base = tcp_buf_discard;
>> -	iov_sock[0].iov_len = already_sent;
>> +	/* Prepare iov according to kernel capability */
>> +	if (!peek_offset_cap) {
>> +		mh_sock.msg_iov = iov_sock;
>> +		iov_sock[0].iov_base = tcp_buf_discard;
>> +		iov_sock[0].iov_len = already_sent;
>> +		mh_sock.msg_iovlen = fill_bufs + 1;
>> +	} else {
>> +		mh_sock.msg_iov = &iov_sock[1];
>> +		mh_sock.msg_iovlen = fill_bufs;
>> +	}
>>   
>>   	if (( v4 && tcp4_payload_used + fill_bufs > TCP_FRAMES_MEM) ||
>>   	    (!v4 && tcp6_payload_used + fill_bufs > TCP_FRAMES_MEM)) {
>> @@ -2279,7 +2309,10 @@ static int tcp_data_from_sock(struct ctx *c, struct tcp_tap_conn *conn)
>>   		return 0;
>>   	}
>>   
>> -	sendlen = len - already_sent;
>> +	sendlen = len;
>> +	if (!peek_offset_cap)
>> +		sendlen -= already_sent;
>> +
>>   	if (sendlen <= 0) {
>>   		conn_flag(c, conn, STALLED);
>>   		return 0;
>> @@ -2449,7 +2482,9 @@ static int tcp_data_from_tap(struct ctx *c, struct tcp_tap_conn *conn,
>>   		flow_trace(conn,
>>   			   "fast re-transmit, ACK: %u, previous sequence: %u",
>>   			   max_ack_seq, conn->seq_to_tap);
>> +
> Spurious change.
>
>>   		conn->seq_to_tap = max_ack_seq;
>> +		tcp_set_peek_offset(conn->sock, 0);
>>   		tcp_data_from_sock(c, conn);
>>   	}
>>   
>> @@ -2542,6 +2577,7 @@ static void tcp_conn_from_sock_finish(struct ctx *c, struct tcp_tap_conn *conn,
>>   	conn->seq_ack_to_tap = conn->seq_from_tap;
>>   
>>   	conn_event(c, conn, ESTABLISHED);
>> +	tcp_set_peek_offset(conn->sock, 0);
>>   
>>   	/* The client might have sent data already, which we didn't
>>   	 * dequeue waiting for SYN,ACK from tap -- check now.
>> @@ -2622,6 +2658,7 @@ int tcp_tap_handler(struct ctx *c, uint8_t pif, sa_family_t af,
>>   			goto reset;
>>   
>>   		conn_event(c, conn, ESTABLISHED);
>> +		tcp_set_peek_offset(conn->sock, 0);
>>   
>>   		if (th->fin) {
>>   			conn->seq_from_tap++;
>> @@ -2788,7 +2825,7 @@ void tcp_listen_handler(struct ctx *c, union epoll_ref ref,
>>   	union sockaddr_inany sa;
>>   	socklen_t sl = sizeof(sa);
>>   	union flow *flow;
>> -	int s;
>> +	int s = 0;
>>   
>>   	if (c->no_tcp || !(flow = flow_alloc()))
>>   		return;
>> @@ -2875,6 +2912,7 @@ void tcp_timer_handler(struct ctx *c, union epoll_ref ref)
>>   			flow_dbg(conn, "ACK timeout, retry");
>>   			conn->retrans++;
>>   			conn->seq_to_tap = conn->seq_ack_from_tap;
>> +			tcp_set_peek_offset(conn->sock, 0);
>>   			tcp_data_from_sock(c, conn);
>>   			tcp_timer_ctl(c, conn);
>>   		}
>> @@ -3166,7 +3204,8 @@ static void tcp_sock_refill_init(const struct ctx *c)
>>    */
>>   int tcp_init(struct ctx *c)
>>   {
>> -	unsigned b;
>> +	unsigned int b, optv = 0;
>> +	int s;
>>   
>>   	for (b = 0; b < TCP_HASH_TABLE_SIZE; b++)
>>   		tc_hash[b] = FLOW_SIDX_NONE;
>> @@ -3190,6 +3229,16 @@ int tcp_init(struct ctx *c)
>>   		NS_CALL(tcp_ns_socks_init, c);
>>   	}
>>   
>> +	/* Probe for SO_PEEK_OFF support */
>> +	s = socket(AF_INET, SOCK_STREAM | SOCK_CLOEXEC, IPPROTO_TCP);
>> +	if (s < 0) {
>> +		warn("Temporary TCP socket creation failed");
>> +	} else {
>> +		if (!setsockopt(s, SOL_SOCKET, SO_PEEK_OFF, &optv, sizeof(int)))
>> +			peek_offset_cap = true;
>> +		close(s);
>> +	}
>> +	info("SO_PEEK_OFF%ssupported", peek_offset_cap ? " " : " not ");
> Maintaining the extra newline before the return would be nice.
There is a reason I prefer printf() when debugging ;-)
I will add it and post a new version shortly.

///jon

> The
> changes actually related to SO_PEEK_OFF look good to me otherwise.
>
>>   	return 0;
>>   }
>>