Re: [PATCH 09/11] udp: Consolidate datagram batching

[Stefano Brivio <sbrivio@redhat.com>]

On Thu,  4 Jul 2024 14:58:33 +1000
David Gibson <david@gibson.dropbear.id.au> wrote:

> When we receive datagrams on a socket, we need to split them into batches
> depending on how they need to be forwarded (either via a specific splice
> socket, or via tap).  The logic to do this, is somewhat awkwardly split
> between udp_buf_sock_handler() itself, udp_splice_send() and
> udp_tap_send().
> 
> Move all the batching logic into udp_buf_sock_handler(), leaving
> udp_splice_send() to just send the prepared batch.  udp_tap_send() reduces
> to just a call to tap_send_frames() so open-code that call in
> udp_buf_sock_handler().
> 
> This will allow separating the batching logic from the rest of the datagram
> forwarding logic, which we'll need for upcoming flow table support.
> 
> Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> ---
>  udp.c | 128 ++++++++++++++++++----------------------------------------
>  1 file changed, 39 insertions(+), 89 deletions(-)
> 
> diff --git a/udp.c b/udp.c
> index 8ed59639..a317e986 100644
> --- a/udp.c
> +++ b/udp.c
> @@ -501,42 +501,29 @@ static void udp_splice_prepare(struct mmsghdr *mmh, unsigned idx)
>  }
>  
>  /**
> - * udp_splice_send() - Send datagrams from socket to socket
> + * udp_splice_send() - Send a batch of datagrams from socket to socket
>   * @c:		Execution context
> - * @start:	Index of first datagram in udp[46]_l2_buf
> - * @n:		Total number of datagrams in udp[46]_l2_buf pool
> - * @dst:	Datagrams will be sent to this port (on destination side)
> + * @start:	Index of batch's first datagram in udp[46]_l2_buf
> + * @n:		Number of datagrams in batch
> + * @src:	Source port for datagram (target side)
> + * @dst:	Destination port for datagrams (target side)
>   * @ref:	epoll reference for origin socket
>   * @now:	Timestamp
> - *
> - * This consumes as many datagrams as are sendable via a single socket.  It
> - * requires that udp_meta[@start].splicesrc is initialised, and will initialise
> - * udp_meta[].splicesrc for each datagram it consumes *and one more* (if
> - * present).
> - *
> - * Return: Number of datagrams forwarded
>   */
> -static unsigned udp_splice_send(const struct ctx *c, size_t start, size_t n,
> -				in_port_t dst, union epoll_ref ref,
> -				const struct timespec *now)
> +static void udp_splice_send(const struct ctx *c, size_t start, size_t n,
> +			    in_port_t src, in_port_t dst,
> +			    union epoll_ref ref,
> +			    const struct timespec *now)
>  {
> -	in_port_t src = udp_meta[start].splicesrc;
> -	struct mmsghdr *mmh_recv;
> -	unsigned int i = start;
>  	int s;
>  
> -	ASSERT(udp_meta[start].splicesrc >= 0);
> -	ASSERT(ref.type == EPOLL_TYPE_UDP);
> -
>  	if (ref.udp.v6) {
> -		mmh_recv = udp6_mh_recv;
>  		udp_spliceto.sa6 = (struct sockaddr_in6) {
>  			.sin6_family = AF_INET6,
>  			.sin6_addr = in6addr_loopback,
>  			.sin6_port = htons(dst),
>  		};
>  	} else {
> -		mmh_recv = udp4_mh_recv;
>  		udp_spliceto.sa4 = (struct sockaddr_in) {
>  			.sin_family = AF_INET,
>  			.sin_addr = in4addr_loopback,
> @@ -544,15 +531,6 @@ static unsigned udp_splice_send(const struct ctx *c, size_t start, size_t n,
>  		};
>  	}
>  
> -	do {
> -		udp_splice_prepare(mmh_recv, i);
> -
> -		if (++i >= n)
> -			break;
> -
> -		udp_meta[i].splicesrc = udp_mmh_splice_port(ref, &mmh_recv[i]);
> -	} while (udp_meta[i].splicesrc == src);
> -
>  	if (ref.udp.pif == PIF_SPLICE) {
>  		src += c->udp.fwd_in.rdelta[src];
>  		s = udp_splice_init[ref.udp.v6][src].sock;
> @@ -560,7 +538,7 @@ static unsigned udp_splice_send(const struct ctx *c, size_t start, size_t n,
>  			s = udp_splice_new(c, ref.udp.v6, src, false);
>  
>  		if (s < 0)
> -			goto out;
> +			return;
>  
>  		udp_splice_ns[ref.udp.v6][dst].ts = now->tv_sec;
>  		udp_splice_init[ref.udp.v6][src].ts = now->tv_sec;
> @@ -577,15 +555,13 @@ static unsigned udp_splice_send(const struct ctx *c, size_t start, size_t n,
>  			s = arg.s;
>  		}
>  		if (s < 0)
> -			goto out;
> +			return;
>  
>  		udp_splice_init[ref.udp.v6][dst].ts = now->tv_sec;
>  		udp_splice_ns[ref.udp.v6][src].ts = now->tv_sec;
>  	}
>  
> -	sendmmsg(s, udp_mh_splice + start, i - start, MSG_NOSIGNAL);
> -out:
> -	return i - start;
> +	sendmmsg(s, udp_mh_splice + start, n, MSG_NOSIGNAL);
>  }
>  
>  /**
> @@ -725,7 +701,7 @@ static size_t udp_update_hdr6(const struct ctx *c,
>   * @v6:		Prepare for IPv6?
>   * @now:	Current timestamp
>   */
> -static void udp_tap_prepare(const struct ctx *c, struct mmsghdr *mmh,
> +static void udp_tap_prepare(const struct ctx *c, const struct mmsghdr *mmh,
>  			    unsigned idx, in_port_t dstport, bool v6,
>  			    const struct timespec *now)
>  {
> @@ -752,49 +728,6 @@ static void udp_tap_prepare(const struct ctx *c, struct mmsghdr *mmh,
>  	(*tap_iov)[UDP_IOV_PAYLOAD].iov_len = l4len;
>  }
>  
> -/**
> - * udp_tap_send() - Prepare UDP datagrams and send to tap interface
> - * @c:		Execution context
> - * @start:	Index of first datagram in udp[46]_l2_buf pool
> - * @n:		Total number of datagrams in udp[46]_l2_buf pool
> - * @dstport:	Destination port number on destination side
> - * @ref:	Epoll reference for origin socket
> - * @now:	Current timestamp
> - *
> - * This consumes as many frames as are sendable via tap.  It requires that
> - * udp_meta[@start].splicesrc is initialised, and will initialise
> - * udp_meta[].splicesrc for each frame it consumes *and one more* (if present).
> - *
> - * Return: Number of frames sent via tap
> - */
> -static unsigned udp_tap_send(const struct ctx *c, size_t start, size_t n,
> -			     in_port_t dstport, union epoll_ref ref,
> -			     const struct timespec *now)
> -{
> -	struct mmsghdr *mmh_recv;
> -	size_t i = start;
> -
> -	ASSERT(udp_meta[start].splicesrc == -1);
> -	ASSERT(ref.type == EPOLL_TYPE_UDP);
> -
> -	if (ref.udp.v6)
> -		mmh_recv = udp6_mh_recv;
> -	else
> -		mmh_recv = udp4_mh_recv;
> -
> -	do {
> -		udp_tap_prepare(c, mmh_recv, i, dstport, ref.udp.v6, now);
> -
> -		if (++i >= n)
> -			break;
> -
> -		udp_meta[i].splicesrc = udp_mmh_splice_port(ref, &mmh_recv[i]);
> -	} while (udp_meta[i].splicesrc == -1);
> -
> -	tap_send_frames(c, &udp_l2_iov[start][0], UDP_NUM_IOVS, i - start);
> -	return i - start;
> -}
> -
>  /**
>   * udp_sock_recv() - Receive datagrams from a socket
>   * @c:		Execution context
> @@ -842,7 +775,7 @@ void udp_buf_sock_handler(const struct ctx *c, union epoll_ref ref, uint32_t eve
>  {
>  	struct mmsghdr *mmh_recv = ref.udp.v6 ? udp6_mh_recv : udp4_mh_recv;
>  	in_port_t dstport = ref.udp.port;
> -	int n, m, i;
> +	int n, i;
>  
>  	if ((n = udp_sock_recv(c, ref.fd, events, mmh_recv)) <= 0)
>  		return;
> @@ -852,19 +785,36 @@ void udp_buf_sock_handler(const struct ctx *c, union epoll_ref ref, uint32_t eve
>  	else if (ref.udp.pif == PIF_HOST)
>  		dstport += c->udp.fwd_in.f.delta[dstport];
>  
> -	/* We divide things into batches based on how we need to send them,
> +	/* We divide datagrams into batches based on how we need to send them,
>  	 * determined by udp_meta[i].splicesrc.  To avoid either two passes
>  	 * through the array, or recalculating splicesrc for a single entry, we
> -	 * have to populate it one entry *ahead* of the loop counter (if
> -	 * present).  So we fill in entry 0 before the loop, then udp_*_send()
> -	 * populate one entry past where they consume.
> +	 * have to populate it one entry *ahead* of the loop counter.
>  	 */
>  	udp_meta[0].splicesrc = udp_mmh_splice_port(ref, mmh_recv);
> -	for (i = 0; i < n; i += m) {
> -		if (udp_meta[i].splicesrc >= 0)
> -			m = udp_splice_send(c, i, n, dstport, ref, now);
> +	for (i = 0; i < n; ) {
> +		int batchsrc = udp_meta[i].splicesrc;
> +		int batchstart = i;
> +
> +		do {
> +			if (batchsrc >= 0)
> +				udp_splice_prepare(mmh_recv, i);
> +			else
> +				udp_tap_prepare(c, mmh_recv, i, dstport,
> +						ref.udp.v6, now);
> +
> +			if (++i >= n)
> +				break;
> +
> +			udp_meta[i].splicesrc = udp_mmh_splice_port(ref,
> +								    &mmh_recv[i]);
> +		} while (udp_meta[i].splicesrc == batchsrc);
> +
> +		if (batchsrc >= 0)
> +			udp_splice_send(c, batchstart, i - batchstart,
> +					batchsrc, dstport, ref, now);
>  		else
> -			m = udp_tap_send(c, i, n, dstport, ref, now);
> +			tap_send_frames(c, &udp_l2_iov[batchstart][0],
> +					UDP_NUM_IOVS, i - batchstart);
>  	}

The logic looks correct to me, but the nested loop makes it a bit hard
to grasp.

I'm wondering if we shouldn't rather have a single loop, always
preparing the datagrams, noting down the previous
udp_meta[i].splicesrc and the first index of a batch, and starting a new
batch (sending the previous one) once the current udp_meta[i].splicesrc
doesn't match the previous value.

I tried to sketch this quickly but failed for the moment to come up
with anything vaguely elegant, so I'm fine with either version.

Nits: curly brackets around multiple lines.

-- 
Stefano