netdev - Re: [PATCH net-next] net: call cond_resched() less often in __release

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAAVpQUCsJGQbDyaB1c3gbz7RQi5KdwgXw1McD82VkxfMT-ewdg@mail.gmail.com>
Date: Wed, 3 Sep 2025 11:27:31 -0700
From: Kuniyuki Iwashima <kuniyu@...gle.com>
To: Eric Dumazet <edumazet@...gle.com>
Cc: "David S . Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>, Neal Cardwell <ncardwell@...gle.com>, 
	Simon Horman <horms@...nel.org>, netdev@...r.kernel.org, eric.dumazet@...il.com
Subject: Re: [PATCH net-next] net: call cond_resched() less often in __release_sock()

On Wed, Sep 3, 2025 at 10:48 AM Eric Dumazet <edumazet@...gle.com> wrote:
>
> 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>

Reviewed-by: Kuniyuki Iwashima <kuniyu@...gle.com>

Thank you!