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>
diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index a697f14..1b68548 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -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: