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

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Message-ID: <20211116072735.68c104ee@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Tue, 16 Nov 2021 07:27:35 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Eric Dumazet <edumazet@...gle.com>
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, 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 ?