Re: [PATCH v4 2/3] tcp: leverage support of SO_PEEK_OFF socket option when available

[Jon Maloy <jmaloy@redhat.com>]

On 2024-05-15 22:29, David Gibson wrote:
> On Wed, May 15, 2024 at 11:34:28AM -0400, Jon Maloy 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>
> I'm pretty sure this needs one more call to tcp_set_peek_offset()
> inside the revert function introduced in the previous patch.
> Otherwise looks good.
Of course. I must have deleted it by accident when I removed the 'paranoid´
test  SEQ_GE(conn->seq_to_tap, equal conn->seq_ack_from_tap).
It worked in my test simply because this never happens. I actually had 
to test the
logics in a separate program.
Thank you for spotting this.

///jon
>> ---
>> 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
>>
>> v4: - Some small changes based on feedback from Stefan/David.
>> ---
>>   tcp.c | 58 ++++++++++++++++++++++++++++++++++++++++++++++++++--------
>>   1 file changed, 50 insertions(+), 8 deletions(-)
>>
>> diff --git a/tcp.c b/tcp.c
>> index 976dba8..4163bf9 100644
>> --- a/tcp.c
>> +++ b/tcp.c
>> @@ -511,6 +511,9 @@ 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];
>>   
>> @@ -526,6 +529,20 @@ 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 to %u in socket %i", offset, s);
>> +}
>> +
>>   /**
>>    * tcp_conn_epoll_events() - epoll events mask for given connection state
>>    * @events:	Current connection events
>> @@ -2197,14 +2214,15 @@ static int tcp_data_from_sock(struct ctx *c, struct tcp_tap_conn *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;
>> -
>>   	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) {
>> @@ -2222,11 +2240,16 @@ static int tcp_data_from_sock(struct ctx *c, struct tcp_tap_conn *conn)
>>   		iov_rem = (wnd_scaled - already_sent) % 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)) {
>> @@ -2267,7 +2290,10 @@ 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;
>> @@ -2438,6 +2464,7 @@ static int tcp_data_from_tap(struct ctx *c, struct tcp_tap_conn *conn,
>>   			   "fast re-transmit, ACK: %u, previous sequence: %u",
>>   			   max_ack_seq, conn->seq_to_tap);
>>   		conn->seq_to_tap = max_ack_seq;
>> +		tcp_set_peek_offset(conn->sock, 0);
>>   		tcp_data_from_sock(c, conn);
>>   	}
>>   
>> @@ -2530,6 +2557,7 @@ 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.
>> @@ -2610,6 +2638,7 @@ 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++;
>> @@ -2863,6 +2892,7 @@ 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);
>>   		}
>> @@ -3154,7 +3184,8 @@ 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;
>> @@ -3178,6 +3209,17 @@ 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 ");
>> +
>>   	return 0;
>>   }
>>