Commit 33946518 authored Feb 14, 2020 by Arjun Roy Committed by David S. Miller Feb 16, 2020

tcp-zerocopy: Return sk_err (if set) along with tcp receive zerocopy.



This patchset is intended to reduce the number of extra system calls
imposed by TCP receive zerocopy. For ping-pong RPC style workloads,
this patchset has demonstrated a system call reduction of about 30%
when coupled with userspace changes.

For applications using epoll, returning sk_err along with the result
of tcp receive zerocopy could remove the need to call
recvmsg()=-EAGAIN after a spurious wakeup.

Consider a multi-threaded application using epoll. A thread may awaken
with EPOLLIN but another thread may already be reading. The
spuriously-awoken thread does not necessarily know that another thread
'won'; rather, it may be possible that it was woken up due to the
presence of an error if there is no data. A zerocopy read receiving 0
bytes thus would need to be followed up by recvmsg to be sure.

Instead, we return sk_err directly with zerocopy, so the application
can avoid this extra system call.

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: David S. Miller <davem@davemloft.net>

parent c8856c05

include/uapi/linux/tcp.h

+1 −0

Original line number	Diff line number	Diff line
		@@ -346,5 +346,6 @@ struct tcp_zerocopy_receive {
		__u32 length; /* in/out: number of bytes to map/mapped */
		__u32 recv_skip_hint; /* out: amount of bytes to skip */
		__u32 inq; /* out: amount of bytes in read queue */
		__s32 err; /* out: socket error */
		};
		#endif /* _UAPI_LINUX_TCP_H */

net/ipv4/tcp.c

+7 −1

Original line number	Diff line number	Diff line
		@@ -3676,14 +3676,20 @@ static int do_tcp_getsockopt(struct sock *sk, int level,
		lock_sock(sk);
		err = tcp_zerocopy_receive(sk, &zc);
		release_sock(sk);
		if (len == sizeof(zc))
		goto zerocopy_rcv_sk_err;
		switch (len) {
		case sizeof(zc):
		case offsetofend(struct tcp_zerocopy_receive, err):
		goto zerocopy_rcv_sk_err;
		case offsetofend(struct tcp_zerocopy_receive, inq):
		goto zerocopy_rcv_inq;
		case offsetofend(struct tcp_zerocopy_receive, length):
		default:
		goto zerocopy_rcv_out;
		}
		zerocopy_rcv_sk_err:
		if (!err)
		zc.err = sock_error(sk);
		zerocopy_rcv_inq:
		zc.inq = tcp_inq_hint(sk);
		zerocopy_rcv_out:

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