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

[David Gibson <david@gibson.dropbear.id.au>]

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On Fri, Jul 05, 2024 at 11:10:45AM +0200, Stefano Brivio wrote:
> 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 don't disagree it's pretty hard to follow, but I haven't really seen
a better way.

> 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 can't really picture what you have in mind here.

> 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.

That, at least, I can fix.

-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson

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