[PATCH net-next 2/2] tcp-zerocopy: Return sk_err (if set) along with tcp receive zerocopy.

[Arjun Roy <arjunroy.kdev@gmail.com>]

From: Arjun Roy <arjunroy@google.com>

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>

---
 include/uapi/linux/tcp.h | 1 +
 net/ipv4/tcp.c           | 8 +++++++-
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/include/uapi/linux/tcp.h b/include/uapi/linux/tcp.h
index 19700101cbba..e1706a7c9d88 100644
--- a/include/uapi/linux/tcp.h
+++ b/include/uapi/linux/tcp.h
@@ -344,5 +344,6 @@ 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 */
diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 947be81b35c5..0efac228bbdb 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -3667,14 +3667,20 @@ 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:
-- 
2.25.0.265.gbab2e86ba0-goog