Re: [PATCH 3/4] udp: Rework how we divide queued datagrams between sending methods

[Stefano Brivio <sbrivio@redhat.com>]

On Wed,  5 Jun 2024 11:39:02 +1000
David Gibson <david@gibson.dropbear.id.au> wrote:

> udp_sock_handler() takes a number of datagrams from sockets that depending
> on their addresses could be forwarded either to the L2 interface ("tap")
> or to another socket ("spliced").  In the latter case we can also only
> send packets together if they have the same source port, and therefore
> are sent via the same socket.
> 
> To reduce the total number of system calls we gather contiguous batches of
> datagrams with the same destination interface and socket where applicable.
> The determination of what the target is is made by udp_mmh_splice_port().
> It returns the source port for splice packets and -1 for "tap" packets.
> We find batches by looking ahead in our queue until we find a datagram
> whose "splicefrom" port doesn't match the first in our current batch.
> 
> udp_mmh_splice_port() is moderately expensive, since it must examine IPv6
> addresses.  But unfortunately we can call it twice on the same datagram:
> once as the (last + 1) entry in one batch (showing that it's not in that
> match, then again as the first entry in the next batch.

This paragraph took me an embarrassingly long time to grasp, if you
re-spin it would be nice to fix it:

- "And unfortunately [...]", I guess: otherwise it looks like we're
  lucky that udp_mmh_splice_port() is expensive or something like that
  (because of the "But" implying contrast).

  I initially assumed "unfortunately" was a typo and tried to
  understand why it was a good thing we'd call udp_mmh_splice_port()
  twice on the same datagram (faster than calling it on two
  datagrams!), then started reading the change and got even more
  confused...

- "(to check that it's not that batch)" ?

> Avoid this by keeping track of the "splice port" in the metadata structure,
> and filling it in one entry ahead of the one we're currently considering.
> This is a bit subtle, but not that hard.  It will also generalise better
> when we have more complex possibilities based on the flow table.

I guess this is the actual, main reason for this change. :) I should
have read this paragraph first.

> 
> Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> ---
>  udp.c | 147 ++++++++++++++++++++++++++++++++++------------------------
>  1 file changed, 86 insertions(+), 61 deletions(-)
> 
> diff --git a/udp.c b/udp.c
> index 7487d2b2..757c10ab 100644
> --- a/udp.c
> +++ b/udp.c
> @@ -198,6 +198,7 @@ static struct ethhdr udp6_eth_hdr;
>   * @ip4h:	Pre-filled IPv4 header (except for tot_len and saddr)
>   * @taph:	Tap backend specific header
>   * @s_in:	Source socket address, filled in by recvmmsg()
> + * @splicesrc:	Source port for splicing, or -1 if not spliceable
>   */
>  static struct udp_meta_t {
>  	struct ipv6hdr ip6h;
> @@ -205,6 +206,7 @@ static struct udp_meta_t {
>  	struct tap_hdr taph;
>  
>  	union sockaddr_inany s_in;
> +	int splicesrc;
>  }
>  #ifdef __AVX2__
>  __attribute__ ((aligned(32)))
> @@ -492,28 +494,32 @@ static int udp_mmh_splice_port(union udp_epoll_ref uref,
>  }
>  
>  /**
> - * udp_splice_sendfrom() - Send datagrams from given port to given port
> + * udp_splice_send() - Send datagrams from socket to socket
>   * @c:		Execution context
>   * @start:	Index of first datagram in udp[46]_l2_buf
> - * @n:		Number of datagrams to send
> - * @src:	Datagrams will be sent from this port (on origin side)
> - * @dst:	Datagrams will be send to this port (on destination side)
> - * @from_pif:	pif from which the packet originated
> - * @v6:		Send as IPv6?
> - * @allow_new:	If true create sending socket if needed, if false discard
> - *              if no sending socket is available
> + * @n:		Total number of datagrams in udp[46]_l2_buf pool
> + * @dst:	Datagrams will be sent to this port (on destination side)
> + * @uref:	UDP epoll reference for origin socket
>   * @now:	Timestamp
> + *
> + * This consumes as many frames as are sendable via a single socket.  It

s/frames/datagrams/ ...or messages.

> + * 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

I'd say it's rather the number of datagrams (not frames) we tried to
send.

In some sense, it's also the number of frames sent _by us_ (well, after
calling sendmmsg(), messages were sent), but we call sendmmsg()
ignoring the result, so this comment might look a bit misleading.

>   */
> -static void udp_splice_sendfrom(const struct ctx *c, unsigned start, unsigned n,
> -				in_port_t src, in_port_t dst, uint8_t from_pif,
> -				bool v6, bool allow_new,
> +static unsigned udp_splice_send(const struct ctx *c, size_t start, size_t n,
> +				in_port_t dst, union udp_epoll_ref uref,
>  				const struct timespec *now)
>  {
> +	in_port_t src = udp_meta[start].splicesrc;
>  	struct mmsghdr *mmh_recv, *mmh_send;
> -	unsigned int i;
> +	unsigned int i = start;
>  	int s;
>  
> -	if (v6) {
> +	ASSERT(udp_meta[start].splicesrc >= 0);
> +
> +	if (uref.v6) {
>  		mmh_recv = udp6_l2_mh_sock;
>  		mmh_send = udp6_mh_splice;
>  	} else {
> @@ -521,40 +527,48 @@ static void udp_splice_sendfrom(const struct ctx *c, unsigned start, unsigned n,
>  		mmh_send = udp4_mh_splice;
>  	}
>  
> -	if (from_pif == PIF_SPLICE) {
> +	do {
> +		mmh_send[i].msg_hdr.msg_iov->iov_len = mmh_recv[i].msg_len;
> +
> +		if (++i >= n)
> +			break;
> +
> +		udp_meta[i].splicesrc = udp_mmh_splice_port(uref, &mmh_recv[i]);
> +	} while (udp_meta[i].splicesrc == src);

I don't have a strong preference, but a for loop like this:

	for (; i < n && udp_meta[i].splicesrc == src; i++) {
		mmh_send[i].msg_hdr.msg_iov->iov_len = mmh_recv[i].msg_len;
		udp_meta[i].splicesrc = udp_mmh_splice_port(uref, &mmh_recv[i]);
	}

	if (i++ < n)	/* Set splicesrc for first mismatching entry, too */
		udp_meta[i].splicesrc = udp_mmh_splice_port(uref, &mmh_recv[i]);

looks a bit more readable to me. Same for udp_tap_send().

> +
> +	if (uref.pif == PIF_SPLICE) {
>  		src += c->udp.fwd_in.rdelta[src];
> -		s = udp_splice_init[v6][src].sock;
> -		if (s < 0 && allow_new)
> -			s = udp_splice_new(c, v6, src, false);
> +		s = udp_splice_init[uref.v6][src].sock;
> +		if (s < 0 && uref.orig)
> +			s = udp_splice_new(c, uref.v6, src, false);
>  
>  		if (s < 0)
> -			return;
> +			goto out;
>  
> -		udp_splice_ns[v6][dst].ts = now->tv_sec;
> -		udp_splice_init[v6][src].ts = now->tv_sec;
> +		udp_splice_ns[uref.v6][dst].ts = now->tv_sec;
> +		udp_splice_init[uref.v6][src].ts = now->tv_sec;
>  	} else {
> -		ASSERT(from_pif == PIF_HOST);
> +		ASSERT(uref.pif == PIF_HOST);
>  		src += c->udp.fwd_out.rdelta[src];
> -		s = udp_splice_ns[v6][src].sock;
> -		if (s < 0 && allow_new) {
> +		s = udp_splice_ns[uref.v6][src].sock;
> +		if (s < 0 && uref.orig) {
>  			struct udp_splice_new_ns_arg arg = {
> -				c, v6, src, -1,
> +				c, uref.v6, src, -1,
>  			};
>  
>  			NS_CALL(udp_splice_new_ns, &arg);
>  			s = arg.s;
>  		}
>  		if (s < 0)
> -			return;
> +			goto out;
>  
> -		udp_splice_init[v6][dst].ts = now->tv_sec;
> -		udp_splice_ns[v6][src].ts = now->tv_sec;
> +		udp_splice_init[uref.v6][dst].ts = now->tv_sec;
> +		udp_splice_ns[uref.v6][src].ts = now->tv_sec;
>  	}
>  
> -	for (i = start; i < start + n; i++)
> -		mmh_send[i].msg_hdr.msg_iov->iov_len = mmh_recv[i].msg_len;
> -
> -	sendmmsg(s, mmh_send + start, n, MSG_NOSIGNAL);
> +	sendmmsg(s, mmh_send + start, i - start, MSG_NOSIGNAL);
> +out:
> +	return i - start;
>  }
>  
>  /**
> @@ -687,31 +701,41 @@ static size_t udp_update_hdr6(const struct ctx *c,
>   * 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:		Number of datagrams to send
> - * @dstport:	Destination port number
> - * @v6:		True if using IPv6
> + * @n:		Total number of datagrams in udp[46]_l2_buf pool
> + * @dstport:	Destination port number on destination side
> + * @uref:	UDP epoll reference for origin socket
>   * @now:	Current timestamp
>   *
> - * Return: size of tap frame with headers
> + * 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 void udp_tap_send(const struct ctx *c,
> -			 unsigned int start, unsigned int n,
> -			 in_port_t dstport, bool v6, const struct timespec *now)
> +static unsigned udp_tap_send(const struct ctx *c, size_t start, size_t n,
> +			     in_port_t dstport, union udp_epoll_ref uref,
> +			     const struct timespec *now)
>  {
>  	struct iovec (*tap_iov)[UDP_NUM_IOVS];
> -	size_t i;
> +	struct mmsghdr *mmh_recv;
> +	size_t i = start;
>  
> -	if (v6)
> +	ASSERT(udp_meta[start].splicesrc == -1);
> +
> +	if (uref.v6) {
>  		tap_iov = udp6_l2_iov_tap;
> -	else
> +		mmh_recv = udp6_l2_mh_sock;
> +	} else {
> +		mmh_recv = udp4_l2_mh_sock;
>  		tap_iov = udp4_l2_iov_tap;
> +	}
>  
> -	for (i = start; i < start + n; i++) {
> +	do {
>  		struct udp_payload_t *bp = &udp_payload[i];
>  		struct udp_meta_t *bm = &udp_meta[i];
>  		size_t l4len;
>  
> -		if (v6) {
> +		if (uref.v6) {
>  			l4len = udp_update_hdr6(c, bm, bp, dstport,
>  						udp6_l2_mh_sock[i].msg_len, now);
>  		} else {
> @@ -719,9 +743,15 @@ static void udp_tap_send(const struct ctx *c,
>  						udp4_l2_mh_sock[i].msg_len, now);
>  		}
>  		tap_iov[i][UDP_IOV_PAYLOAD].iov_len = l4len;
> -	}
>  
> -	tap_send_frames(c, &tap_iov[start][0], UDP_NUM_IOVS, n);
> +		if (++i >= n)
> +			break;
> +
> +		udp_meta[i].splicesrc = udp_mmh_splice_port(uref, &mmh_recv[i]);
> +	} while (udp_meta[i].splicesrc == -1);
> +
> +	tap_send_frames(c, &tap_iov[start][0], UDP_NUM_IOVS, i - start);
> +	return i - start;
>  }
>  
>  /**
> @@ -770,24 +800,19 @@ void udp_sock_handler(const struct ctx *c, union epoll_ref ref, uint32_t events,
>  	if (n <= 0)
>  		return;
>  
> +	/* We divide things 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.
> +	 */
> +	udp_meta[0].splicesrc = udp_mmh_splice_port(ref.udp, mmh_recv);
>  	for (i = 0; i < n; i += m) {
> -		int splicefrom = -1;
> -
> -		splicefrom = udp_mmh_splice_port(ref.udp, mmh_recv + i);
> -
> -		for (m = 1; i + m < n; m++) {
> -			int p;
> -
> -			p = udp_mmh_splice_port(ref.udp, mmh_recv + i + m);
> -			if (p != splicefrom)
> -				break;
> -		}
> -
> -		if (splicefrom >= 0)
> -			udp_splice_sendfrom(c, i, m, splicefrom, dstport,
> -					    ref.udp.pif, v6, ref.udp.orig, now);
> +		if (udp_meta[i].splicesrc >= 0)
> +			m = udp_splice_send(c, i, n, dstport, ref.udp, now);
>  		else
> -			udp_tap_send(c, i, m, dstport, v6, now);
> +			m = udp_tap_send(c, i, n, dstport, ref.udp, now);
>  	}
>  }
>  

The rest of the series looks good to me.

-- 
Stefano