netdev - [PATCH net-next 2/3] tcp: add a fast path in tcp_write

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Message-ID: <20241002173042.917928-3-edumazet@google.com>
Date: Wed,  2 Oct 2024 17:30:41 +0000
From: Eric Dumazet <edumazet@...gle.com>
To: "David S . Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>
Cc: Neal Cardwell <ncardwell@...gle.com>, netdev@...r.kernel.org, eric.dumazet@...il.com, 
	Eric Dumazet <edumazet@...gle.com>
Subject: [PATCH net-next 2/3] tcp: add a fast path in tcp_write_timer()

retransmit timer is not stopped from inet_csk_clear_xmit_timer()
because we do not define INET_CSK_CLEAR_TIMERS.

This is a conscious choice : for active TCP flows, it is better
to only call mod_timer(), because there is more chances of
keeping the timer unchanged. Also inet_csk_clear_xmit_timer()
is often called from another cpu, and calling del_timer()
would cause false sharing and lock contention.

This means that very often, tcp_write_timer() is called
at the timer expiration, while there is nothing to retransmit.

This can be detected very early, avoiding the socket spinlock.

Signed-off-by: Eric Dumazet <edumazet@...gle.com>
---
 net/ipv4/tcp_timer.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
index 56c597e763ac7a8cebeba324f84e57b1eeeae977..b7266b9101ce5933776bd38d086287667e3a7f18 100644
--- a/net/ipv4/tcp_timer.c
+++ b/net/ipv4/tcp_timer.c
@@ -717,6 +717,10 @@ static void tcp_write_timer(struct timer_list *t)
 			from_timer(icsk, t, icsk_retransmit_timer);
 	struct sock *sk = &icsk->icsk_inet.sk;

+	/* Avoid locking the socket when there is no pending event. */
+	if (!smp_load_acquire(&icsk->icsk_pending))
+		goto out;
+
 	bh_lock_sock(sk);
 	if (!sock_owned_by_user(sk)) {
 		tcp_write_timer_handler(sk);
@@ -726,6 +730,7 @@ static void tcp_write_timer(struct timer_list *t)
 			sock_hold(sk);
 	}
 	bh_unlock_sock(sk);
+out:
 	sock_put(sk);
 }

-- 
2.47.0.rc0.187.ge670bccf7e-goog