Commit 16c61016 authored Sep 03, 2025 by Eric Dumazet Committed by Jakub Kicinski Sep 04, 2025

net: call cond_resched() less often in __release_sock()

While stress testing TCP I had unexpected retransmits and sack packets
when a single cpu receives data from multiple high-throughput flows.

super_netperf 4 -H srv -T,10 -l 3000 &

Tcpdump extract:

00:00:00.000007 IP6 clnt > srv: Flags [.], seq 26062848:26124288, ack 1, win 66, options [nop,nop,TS val 651460834 ecr 3100749131], length 61440
00:00:00.000006 IP6 clnt > srv: Flags [.], seq 26124288:26185728, ack 1, win 66, options [nop,nop,TS val 651460834 ecr 3100749131], length 61440
00:00:00.000005 IP6 clnt > srv: Flags [P.], seq 26185728:26243072, ack 1, win 66, options [nop,nop,TS val 651460834 ecr 3100749131], length 57344
00:00:00.000006 IP6 clnt > srv: Flags [.], seq 26243072:26304512, ack 1, win 66, options [nop,nop,TS val 651460844 ecr 3100749141], length 61440
00:00:00.000005 IP6 clnt > srv: Flags [.], seq 26304512:26365952, ack 1, win 66, options [nop,nop,TS val 651460844 ecr 3100749141], length 61440
00:00:00.000007 IP6 clnt > srv: Flags [P.], seq 26365952:26423296, ack 1, win 66, options [nop,nop,TS val 651460844 ecr 3100749141], length 57344
00:00:00.000006 IP6 clnt > srv: Flags [.], seq 26423296:26484736, ack 1, win 66, options [nop,nop,TS val 651460853 ecr 3100749150], length 61440
00:00:00.000005 IP6 clnt > srv: Flags [.], seq 26484736:26546176, ack 1, win 66, options [nop,nop,TS val 651460853 ecr 3100749150], length 61440
00:00:00.000005 IP6 clnt > srv: Flags [P.], seq 26546176:26603520, ack 1, win 66, options [nop,nop,TS val 651460853 ecr 3100749150], length 57344
00:00:00.003932 IP6 clnt > srv: Flags [P.], seq 26603520:26619904, ack 1, win 66, options [nop,nop,TS val 651464844 ecr 3100753141], length 16384
00:00:00.006602 IP6 clnt > srv: Flags [.], seq 24862720:24866816, ack 1, win 66, options [nop,nop,TS val 651471419 ecr 3100759716], length 4096
00:00:00.013000 IP6 clnt > srv: Flags [.], seq 24862720:24866816, ack 1, win 66, options [nop,nop,TS val 651484421 ecr 3100772718], length 4096
00:00:00.000416 IP6 srv > clnt: Flags [.], ack 26619904, win 1393, options [nop,nop,TS val 3100773185 ecr 651484421,nop,nop,sack 1 {24862720:24866816}], length 0

After analysis, it appears this is because of the cond_resched()
call from __release_sock().

When current thread is yielding, while still holding the TCP socket lock,
it might regain the cpu after a very long time.

Other peer TLP/RTO is firing (multiple times) and packets are retransmit,
while the initial copy is waiting in the socket backlog or receive queue.

In this patch, I call cond_resched() only once every 16 packets.

Modern TCP stack now spends less time per packet in the backlog,
especially because ACK are no longer sent (commit 133c4c0d
"tcp: defer regular ACK while processing socket backlog")

Before:

After:

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20250903174811.1930820-1-edumazet@google.com

Signed-off-by: Jakub Kicinski <kuba@kernel.org>

parent abfa70b3

net/core/sock.c

+8 −4

Original line number	Diff line number	Diff line
		@@ -3165,23 +3165,27 @@ void __release_sock(struct sock *sk)
		__acquires(&sk->sk_lock.slock)
		{
		struct sk_buff skb, next;
		int nb = 0;

		while ((skb = sk->sk_backlog.head) != NULL) {
		sk->sk_backlog.head = sk->sk_backlog.tail = NULL;

		spin_unlock_bh(&sk->sk_lock.slock);

		do {
		while (1) {
		next = skb->next;
		prefetch(next);
		DEBUG_NET_WARN_ON_ONCE(skb_dst_is_noref(skb));
		skb_mark_not_on_list(skb);
		sk_backlog_rcv(sk, skb);

		cond_resched();

		skb = next;
		} while (skb != NULL);
		if (!skb)
		break;

		if (!(++nb & 15))
		cond_resched();
		}

		spin_lock_bh(&sk->sk_lock.slock);
		}