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

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Message-ID: <CAM_iQpWah2M2tG=+eRS86VtjknTiBC42DSwdHB8USpXgRsfWjw@mail.gmail.com>
Date:   Wed, 23 Oct 2019 10:40:03 -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 7:15 PM Eric Dumazet <edumazet@...gle.com> wrote:
>
> On Tue, Oct 22, 2019 at 6:10 PM Cong Wang <xiyou.wangcong@...il.com> wrote:
> >
> > 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.
>
> How many TX/RX queues on the NIC ?

60 queues (default), 25Gbps NIC, mlx5.

> What is the qdisc setup ?

fq_codel, which is default here. Its parameters are default too.

>
> >
> > 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.
>
> OK thanks for using something I can repro easily :)
>
> I ran the experiment ten times :

How many CPU's do you have?


>
> lpaa23:/export/hda3/google/edumazet# echo 3
> >/proc/sys/net/ipv4/tcp_early_retrans
> lpaa23:/export/hda3/google/edumazet# for f in {1..10}; do
> ./super_netperf 1 -H lpaa24 -t TCP_RR -l 20; done
>   26797
>   26850
>   25266
>   27605
>   26586
>   26341
>   27255
>   27532
>   26657
>   27253
>
>
> Then disabled tlp, and got no obvious difference
>
> lpaa23:/export/hda3/google/edumazet# echo 0
> >/proc/sys/net/ipv4/tcp_early_retrans
> lpaa23:/export/hda3/google/edumazet# for f in {1..10}; do
> ./super_netperf 1 -H lpaa24 -t TCP_RR -l 20; done
>   25311
>   24658
>   27105
>   27421
>   27604
>   24649
>   26259
>   27615
>   27543
>   26217
>
> I tried with 256 concurrent flows, and same overall observation about
> tlp not changing the numbers.
> (In fact I am not even sure we arm RTO at all while doing a TCP_RR)

In case you misunderstand, the CPU profiling I used is captured
during 256 parallel TCP_STREAM.


> lpaa23:/export/hda3/google/edumazet# echo 3
> >/proc/sys/net/ipv4/tcp_early_retrans
> lpaa23:/export/hda3/google/edumazet# for f in {1..10}; do
> ./super_netperf 256 -H lpaa24 -t TCP_RR -l 20; done
> 1578682
> 1572444
> 1573490
> 1536378
> 1514905
> 1580854
> 1575949
> 1578925
> 1511164
> 1568213
> lpaa23:/export/hda3/google/edumazet# echo 0
> >/proc/sys/net/ipv4/tcp_early_retrans
> lpaa23:/export/hda3/google/edumazet# for f in {1..10}; do
> ./super_netperf 256 -H lpaa24 -t TCP_RR -l 20; done
> 1576228
> 1578401
> 1577654
> 1579506
> 1570682
> 1582267
> 1550069
> 1530599
> 1583269
> 1578830
>
>
> I wonder if you have some IRQ smp_affinity problem maybe, or some
> scheduler strategy constantly migrating your user threads ?

Scheduler is default too. IRQ smp affinity is statically distributed to
each of the first 60 CPU's.

>
> TLP is quite subtle, having two timers instead of one is probably
> going to trigger various bugs.

This is exactly why I keep icsk_pending. ;)

Thanks.