tcp: remove one ktime_get() from recvmsg() fast path

Each time some payload is consumed by user space (recvmsg() and friends),
TCP calls tcp_rcv_space_adjust() to run DRS algorithm to check
if an increase of sk->sk_rcvbuf is needed.

This function is based on time sampling, and currently calls
tcp_mstamp_refresh(tp), which is a wrapper around ktime_get_ns().

ktime_get_ns() has a high cost on some platforms.
100+ cycles for rdtscp on AMD EPYC Turin for instance.

We do not have to refresh tp->tcp_mpstamp, using the last cached value
is enough. We only need to refresh it from __tcp_cleanup_rbuf()
if an ACK must be sent (this is a rare event).

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20251024120707.3516550-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index b79da6d39392751e189f1f65969b15c904a6792a..a9345aa5a2e5f4a2ca7ca599e7523d017ffa64ee 100644 (file)
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -1556,8 +1556,10 @@ void __tcp_cleanup_rbuf(struct sock *sk, int copied)
                                time_to_ack = true;
                }
        }
-       if (time_to_ack)
+       if (time_to_ack) {
+               tcp_mstamp_refresh(tp);
                tcp_send_ack(sk);
+       }
 }
 
 void tcp_cleanup_rbuf(struct sock *sk, int copied)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 8fc97f4d8a6b2f8e39cabf6c9b3e6cdae294a5f5..ff19f6e54d55cb63f04c2da0b241e3d7d2f946a0 100644 (file)
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -928,9 +928,15 @@ void tcp_rcv_space_adjust(struct sock *sk)
 
        trace_tcp_rcv_space_adjust(sk);
 
-       tcp_mstamp_refresh(tp);
+       if (unlikely(!tp->rcv_rtt_est.rtt_us))
+               return;
+
+       /* We do not refresh tp->tcp_mstamp here.
+        * Some platforms have expensive ktime_get() implementations.
+        * Using the last cached value is enough for DRS.
+        */
        time = tcp_stamp_us_delta(tp->tcp_mstamp, tp->rcvq_space.time);
-       if (time < (tp->rcv_rtt_est.rtt_us >> 3) || tp->rcv_rtt_est.rtt_us == 0)
+       if (time < (tp->rcv_rtt_est.rtt_us >> 3))
                return;
 
        /* Number of bytes copied to user in last RTT */