tcp: improve latencies of timer triggered events

Modern TCP stack highly depends on tcp_write_timer() having a small
latency, but current implementation doesn't exactly meet the
expectations.

When a timer fires but finds the socket is owned by the user, it rearms
itself for an additional delay hoping next run will be more
successful.

tcp_write_timer() for example uses a 50ms delay for next try, and it
defeats many attempts to get predictable TCP behavior in term of
latencies.

Use the recently introduced tcp_release_cb(), so that the user owning
the socket will call various handlers right before socket release.

This will permit us to post a followup patch to address the
tcp_tso_should_defer() syndrome (some deferred packets have to wait
RTO timer to be transmitted, while cwnd should allow us to send them
sooner)

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Tom Herbert <therbert@google.com>
Cc: Yuchung Cheng <ycheng@google.com>
Cc: Neal Cardwell <ncardwell@google.com>
Cc: Nandita Dukkipati <nanditad@google.com>
Cc: H.K. Jerry Chu <hkchu@google.com>
Cc: John Heffner <johnwheffner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 3 insertions, 1 deletions

diff --git a/include/linux/tcp.h b/include/linux/tcp.h
index 9febfb685c33..2761856987b2 100644
--- a/include/linux/tcp.h
+++ b/include/linux/tcp.h
@@ -515,7 +515,9 @@ struct tcp_sock {
 enum tsq_flags {
         TSQ_THROTTLED,
         TSQ_QUEUED,
-        TSQ_OWNED, /* tcp_tasklet_func() found socket was locked */
+        TCP_TSQ_DEFERRED,          /* tcp_tasklet_func() found socket was owned */
+        TCP_WRITE_TIMER_DEFERRED,  /* tcp_write_timer() found socket was owned */
+        TCP_DELACK_TIMER_DEFERRED, /* tcp_delack_timer() found socket was owned */
 };
 
 static inline struct tcp_sock *tcp_sk(const struct sock *sk)