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

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Message-ID: <CANn89iJb7s-JoCCfn=eoxZ_tX_2RaeEPZKO1aHyHtgHxLXsd2A@mail.gmail.com>
Date:   Tue, 16 Nov 2021 08:46:37 -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 7:27 AM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Tue, 16 Nov 2021 07:22:02 -0800 Eric Dumazet wrote:
> > 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
>
> Huh, somehow I assumed your 4k MTU numbers were with zero-copy :o
>
> Out of curiosity - what's the softirq load with 4k? Do you have an
> idea what the load is on the CPU consuming the data vs the softirq
> processing with 1500B ?

On my testing host,

4K MTU : processing ~2,600.000 packets per second in GRO and other parts
use about 60% of the core in BH.
(Some of this cost comes from a clang issue, and the csum_partial() one
I was working on last week)
NIC RX interrupts are firing about 25,000 times per second in this setup.

1500 MTU : processing ~ 5,800,000 packets per second uses one core in
BH (and also one core in recvmsg()),
We stay in NAPI mode (no IRQ rearming)
(That was with a TCP_STREAM run sustaining 70Gbit)

BH numbers also depend on IRQ coalescing parameters.