netdev - Re: [PATCH net-next 17/20] tcp: defer skb freeing after socket lock is released

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Date:   Tue, 16 Nov 2021 07:22:02 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Soheil Hassas Yeganeh <soheil@...gle.com>,
        Neal Cardwell <ncardwell@...gle.com>,
        Arjun Roy <arjunroy@...gle.com>
Subject: Re: [PATCH net-next 17/20] tcp: defer skb freeing after socket lock
 is released

On Tue, Nov 16, 2021 at 7:05 AM Eric Dumazet <edumazet@...gle.com> wrote:
>
> On Tue, Nov 16, 2021 at 6:27 AM Jakub Kicinski <kuba@...nel.org> wrote:
> >
> > On Mon, 15 Nov 2021 11:02:46 -0800 Eric Dumazet wrote:
> > > One cpu can now be fully utilized for the kernel->user copy,
> > > and another cpu is handling BH processing and skb/page
> > > allocs/frees (assuming RFS is not forcing use of a single CPU)
> >
> > Are you saying the kernel->user copy is not under the socket lock
> > today? I'm working on getting the crypto & copy from under the socket
> > lock for ktls, and it looked like tcp does the copy under the lock.
>
> Copy is done currently with socket lock owned.
>
> But each skb is freed one at a time, after its payload has been consumed.
>
> Note that I am also working on performing the copy while still allowing BH
> to process incoming packets.
>
> This is a bit more complex, but I think it is doable.

Here is the perf top profile on cpu used by user thread doing the
recvmsg(), at 96 Gbit/s

We no longer see skb freeing related costs, but we still see costs of
having to process the backlog.

   81.06%  [kernel]       [k] copy_user_enhanced_fast_string
     2.50%  [kernel]       [k] __skb_datagram_iter
     2.25%  [kernel]       [k] _copy_to_iter
     1.45%  [kernel]       [k] tcp_recvmsg_locked
     1.39%  [kernel]       [k] tcp_rcv_established
     0.93%  [kernel]       [k] skb_try_coalesce
     0.79%  [kernel]       [k] sock_rfree
     0.72%  [kernel]       [k] tcp_v6_do_rcv
     0.57%  [kernel]       [k] skb_release_data
     0.50%  [kernel]       [k] tcp_queue_rcv
     0.43%  [kernel]       [k] __direct_call_clocksource_read1
     0.43%  [kernel]       [k] __release_sock
     0.39%  [kernel]       [k] _raw_spin_lock
     0.25%  [kernel]       [k] __direct_call_hrtimer_clock_base_get_time1
     0.20%  [kernel]       [k] __tcp_transmit_skb
     0.19%  [kernel]       [k] __dev_queue_xmit
     0.18%  [kernel]       [k] __tcp_select_window
     0.18%  [kernel]       [k] _raw_spin_lock_bh