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

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Date:   Tue, 22 Oct 2019 16:24:31 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Cong Wang <xiyou.wangcong@...il.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 4:11 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.