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Message-ID: <20250903174811.1930820-1-edumazet@google.com>
Date: Wed, 3 Sep 2025 17:48:10 +0000
From: Eric Dumazet <edumazet@...gle.com>
To: "David S . Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>,
Paolo Abeni <pabeni@...hat.com>, Neal Cardwell <ncardwell@...gle.com>
Cc: Simon Horman <horms@...nel.org>, Kuniyuki Iwashima <kuniyu@...gle.com>, netdev@...r.kernel.org,
eric.dumazet@...il.com, Eric Dumazet <edumazet@...gle.com>
Subject: [PATCH net-next] 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 133c4c0d3717
"tcp: defer regular ACK while processing socket backlog")
Before:
clnt:/# nstat -n;sleep 10;nstat|egrep "TcpOutSegs|TcpRetransSegs|TCPFastRetrans|TCPTimeouts|Probes|TCPSpuriousRTOs|DSACK"
TcpOutSegs 19046186 0.0
TcpRetransSegs 1471 0.0
TcpExtTCPTimeouts 1397 0.0
TcpExtTCPLossProbes 1356 0.0
TcpExtTCPDSACKRecv 1352 0.0
TcpExtTCPSpuriousRTOs 114 0.0
TcpExtTCPDSACKRecvSegs 1352 0.0
After:
clnt:/# nstat -n;sleep 10;nstat|egrep "TcpOutSegs|TcpRetransSegs|TCPFastRetrans|TCPTimeouts|Probes|TCPSpuriousRTOs|DSACK"
TcpOutSegs 19218936 0.0
Signed-off-by: Eric Dumazet <edumazet@...gle.com>
---
net/core/sock.c | 12 ++++++++----
1 file changed, 8 insertions(+), 4 deletions(-)
diff --git a/net/core/sock.c b/net/core/sock.c
index 9a8290fcc35d..0a93443a7402 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -3187,23 +3187,27 @@ 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);
}
--
2.51.0.338.gd7d06c2dae-goog
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