[PATCH v2 net-next 3/8] tcp: tsq: add shortcut in tcp_tasklet_func()

[Eric Dumazet]

Under high stress, I've seen tcp_tasklet_func() consuming
~700 usec, handling ~150 tcp sockets.

By setting TCP_TSQ_DEFERRED in tcp_wfree(), we give a chance
for other cpus/threads entering tcp_write_xmit() to grab it,
allowing tcp_tasklet_func() to skip sockets that already did
an xmit cycle.

In the future, we might give to ACK processing an increased
budget to reduce even more tcp_tasklet_func() amount of work.

Signed-off-by: Eric Dumazet <eduma...@google.com>
---
 net/ipv4/tcp_output.c | 22 ++++++++++++----------
 1 file changed, 12 insertions(+), 10 deletions(-)

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 4adaf8e1bb63..fa23b688a6f3 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -767,19 +767,19 @@ static void tcp_tasklet_func(unsigned long data)
        list_for_each_safe(q, n, &list) {
                tp = list_entry(q, struct tcp_sock, tsq_node);
                list_del(&tp->tsq_node);
+               clear_bit(TSQ_QUEUED, &tp->tsq_flags);
 
                sk = (struct sock *)tp;
-               bh_lock_sock(sk);
-
-               if (!sock_owned_by_user(sk)) {
-                       tcp_tsq_handler(sk);
-               } else {
-                       /* defer the work to tcp_release_cb() */
-                       set_bit(TCP_TSQ_DEFERRED, &tp->tsq_flags);
+               if (!sk->sk_lock.owned &&
+                   test_bit(TCP_TSQ_DEFERRED, &tp->tsq_flags)) {
+                       bh_lock_sock(sk);
+                       if (!sock_owned_by_user(sk)) {
+                               clear_bit(TCP_TSQ_DEFERRED, &tp->tsq_flags);
+                               tcp_tsq_handler(sk);
+                       }
+                       bh_unlock_sock(sk);
                }
-               bh_unlock_sock(sk);
 
-               clear_bit(TSQ_QUEUED, &tp->tsq_flags);
                sk_free(sk);
        }
 }
@@ -884,7 +884,7 @@ void tcp_wfree(struct sk_buff *skb)
                if (!(oval & TSQF_THROTTLED) || (oval & TSQF_QUEUED))
                        goto out;
 
-               nval = (oval & ~TSQF_THROTTLED) | TSQF_QUEUED;
+               nval = (oval & ~TSQF_THROTTLED) | TSQF_QUEUED | 
TCPF_TSQ_DEFERRED;
                nval = cmpxchg(&tp->tsq_flags, oval, nval);
                if (nval != oval)
                        continue;
@@ -2229,6 +2229,8 @@ static bool tcp_write_xmit(struct sock *sk, unsigned int 
mss_now, int nonagle,
                    unlikely(tso_fragment(sk, skb, limit, mss_now, gfp)))
                        break;
 
+               if (test_bit(TCP_TSQ_DEFERRED, &tp->tsq_flags))
+                       clear_bit(TCP_TSQ_DEFERRED, &tp->tsq_flags);
                if (tcp_small_queue_check(sk, skb, 0))
                        break;
 
-- 
2.8.0.rc3.226.g39d4020