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>

1 files changed, 1 insertions, 0 deletions

diff --git a/include/uapi/linux/tcp.h b/include/uapi/linux/tcp.h
index 548f480b9c66..1a7fc856e237 100644
--- a/include/uapi/linux/tcp.h
+++ b/include/uapi/linux/tcp.h
@@ -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 */