Re: [PATCH net-next] tcp: forbid direct reclaim if MSG_DONTWAIT is set in send path

By default, the sk->sk_allocation is GFP_KERNEL, that means if there's
no enough memory it will do both direct reclaim and background reclaim.
If the size of system memory is great, the direct reclaim may cause great
latency spike.

When we set MSG_DONTWAIT in send syscalls, we really don't want it to be
blocked, so we'd better clear __GFP_DIRECT_RECLAIM when allocate skb in the
send path. Then, it will return immediately if there's no enough memory to
be allocated, and then the appliation has a chance to do some other stuffs
instead of being blocked here.

Signed-off-by: Yafang Shao <redacted>
---
 net/ipv4/tcp.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 43ef83b..fe4f5ce 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c

@@ -1182,6 +1182,7 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)
        bool process_backlog = false;
        bool zc = false;
        long timeo;
+       gfp_t gfp;

        flags = msg->msg_flags;

@@ -1255,6 +1256,9 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)
        /* Ok commence sending. */
        copied = 0;

+       gfp = flags & MSG_DONTWAIT ? sk->sk_allocation & ~__GFP_DIRECT_RECLAIM :
+             sk->sk_allocation;
+
 restart:
        mss_now = tcp_send_mss(sk, &size_goal, flags);

@@ -1283,8 +1287,7 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)
                        }
                        first_skb = tcp_rtx_and_write_queues_empty(sk);
                        linear = select_size(first_skb, zc);
-                       skb = sk_stream_alloc_skb(sk, linear, sk->sk_allocation,
-                                                 first_skb);
+                       skb = sk_stream_alloc_skb(sk, linear, gfp, first_skb);
                        if (!skb)
                                goto wait_for_memory;