Commit f21a3c48 authored Dec 02, 2020 by Arjun Roy Committed by Jakub Kicinski Dec 04, 2020

net-zerocopy: Introduce short-circuit small reads.



Sometimes, we may call tcp receive zerocopy when inq is 0,
or inq < PAGE_SIZE, or inq is generally small enough that
it is cheaper to copy rather than remap pages.

In these cases, we may want to either return early (inq=0) or
attempt to use the provided copy buffer to simply copy
the received data.

This allows us to save both system call overhead and
the latency of acquiring mmap_sem in read mode for cases where
it would be useless to do so.

This patchset enables this behaviour by:
1. Returning quickly if inq is 0.
2. Attempting to perform a regular copy if a hybrid copybuffer is
   provided and it is large enough to absorb all available bytes.
3. Return quickly if no such buffer was provided and there are less
   than PAGE_SIZE bytes available.

For small RPC ping-pong workloads, normally we would have
1 getsockopt(), 1 recvmsg() and 1 sendmsg() call per RPC. With this
change, we remove the recvmsg() call entirely, reducing the syscall
overhead by about 33%. In testing with small (hundreds of bytes)
RPC traffic, this yields a syscall reduction of about 33% and
an efficiency gain of about 3-5% when defined as QPS/CPU Util.

Signed-off-by: Arjun Roy <arjunroy@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Soheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

parent 936ced41

net/ipv4/tcp.c

+36 −0

Original line number	Diff line number	Diff line
		@@ -1800,6 +1800,39 @@ static int find_next_mappable_frag(const skb_frag_t *frag,
		return offset;
		}

		static int tcp_recvmsg_locked(struct sock sk, struct msghdr msg, size_t len,
		int nonblock, int flags,
		struct scm_timestamping_internal *tss,
		int *cmsg_flags);
		static int receive_fallback_to_copy(struct sock *sk,
		struct tcp_zerocopy_receive *zc, int inq)
		{
		unsigned long copy_address = (unsigned long)zc->copybuf_address;
		struct scm_timestamping_internal tss_unused;
		int err, cmsg_flags_unused;
		struct msghdr msg = {};
		struct iovec iov;

		zc->length = 0;
		zc->recv_skip_hint = 0;

		if (copy_address != zc->copybuf_address)
		return -EINVAL;

		err = import_single_range(READ, (void __user *)copy_address,
		inq, &iov, &msg.msg_iter);
		if (err)
		return err;

		err = tcp_recvmsg_locked(sk, &msg, inq, /nonblock=/1, /flags=/0,
		&tss_unused, &cmsg_flags_unused);
		if (err < 0)
		return err;

		zc->copybuf_len = err;
		return 0;
		}

		static int tcp_copy_straggler_data(struct tcp_zerocopy_receive *zc,
		struct sk_buff *skb, u32 copylen,
		u32 offset, u32 seq)
		@@ -1904,6 +1937,9 @@ static int tcp_zerocopy_receive(struct sock *sk,

		sock_rps_record_flow(sk);

		if (inq && inq <= copybuf_len)
		return receive_fallback_to_copy(sk, zc, inq);

		if (inq < PAGE_SIZE) {
		zc->length = 0;
		zc->recv_skip_hint = inq;

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