netdev - Re: [Patch net-next 3/3] tcp: decouple TLP timer from RTO timer

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: <CAM_iQpWxDN7Wm_ar7cStiMnhmmy7RK=BG5k4zwD+K6kbgfPEKw@mail.gmail.com>
Date:   Tue, 22 Oct 2019 18:10:27 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     netdev <netdev@...r.kernel.org>, Yuchung Cheng <ycheng@...gle.com>,
        Neal Cardwell <ncardwell@...gle.com>
Subject: Re: [Patch net-next 3/3] tcp: decouple TLP timer from RTO timer

On Tue, Oct 22, 2019 at 4:24 PM Eric Dumazet <edumazet@...gle.com> wrote:
>
> On Tue, Oct 22, 2019 at 4:11 PM Cong Wang <xiyou.wangcong@...il.com> wrote:
> >
> > Currently RTO, TLP and PROBE0 all share a same timer instance
> > in kernel and use icsk->icsk_pending to dispatch the work.
> > This causes spinlock contention when resetting the timer is
> > too frequent, as clearly shown in the perf report:
> >
> >    61.72%    61.71%  swapper          [kernel.kallsyms]                        [k] queued_spin_lock_slowpath
> >    ...
> >     - 58.83% tcp_v4_rcv
> >       - 58.80% tcp_v4_do_rcv
> >          - 58.80% tcp_rcv_established
> >             - 52.88% __tcp_push_pending_frames
> >                - 52.88% tcp_write_xmit
> >                   - 28.16% tcp_event_new_data_sent
> >                      - 28.15% sk_reset_timer
> >                         + mod_timer
> >                   - 24.68% tcp_schedule_loss_probe
> >                      - 24.68% sk_reset_timer
> >                         + 24.68% mod_timer
> >
> > This patch decouples TLP timer from RTO timer by adding a new
> > timer instance but still uses icsk->icsk_pending to dispatch,
> > in order to minimize the risk of this patch.
> >
> > After this patch, the CPU time spent in tcp_write_xmit() reduced
> > down to 10.92%.
>
> What is the exact benchmark you are running ?
>
> We never saw any contention like that, so lets make sure you are not
> working around another issue.

I simply ran 256 parallel netperf with 128 CPU's to trigger this
spinlock contention, 100% reproducible here.

A single netperf TCP_RR could _also_ confirm the improvement:

Before patch:

$ netperf -H XXX -t TCP_RR -l 20
MIGRATED TCP REQUEST/RESPONSE TEST from 0.0.0.0 (0.0.0.0) port 0
AF_INET to XXX () port 0 AF_INET : first burst 0
Local /Remote
Socket Size   Request  Resp.   Elapsed  Trans.
Send   Recv   Size     Size    Time     Rate
bytes  Bytes  bytes    bytes   secs.    per sec

655360 873800 1        1       20.00    17665.59
655360 873800


After patch:

$ netperf -H XXX -t TCP_RR -l 20
MIGRATED TCP REQUEST/RESPONSE TEST from 0.0.0.0 (0.0.0.0) port 0
AF_INET to XXX () port 0 AF_INET : first burst 0
Local /Remote
Socket Size   Request  Resp.   Elapsed  Trans.
Send   Recv   Size     Size    Time     Rate
bytes  Bytes  bytes    bytes   secs.    per sec

655360 873800 1        1       20.00    18829.31
655360 873800

(I have run it for multiple times, just pick a median one here.)

The difference can also be observed by turning off/on TLP without patch.

Thanks.