On Wed, Oct 23, 2019 at 10:40 AM Cong Wang <xiyou.wangc...@gmail.com> wrote:
>
> On Tue, Oct 22, 2019 at 7:15 PM Eric Dumazet <eduma...@google.com> wrote:
> >
> > On Tue, Oct 22, 2019 at 6:10 PM Cong Wang <xiyou.wangc...@gmail.com> wrote:
> > >
> > > On Tue, Oct 22, 2019 at 4:24 PM Eric Dumazet <eduma...@google.com> wrote:
> > > >
> > > > On Tue, Oct 22, 2019 at 4:11 PM Cong Wang <xiyou.wangc...@gmail.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.
When I asked you the workload, you gave me TCP_RR output, not TCP_STREAM :/
"A single netperf TCP_RR could _also_ confirm the improvement:"
