linux-kernel - Re: [PATCH] fs: eventpoll: add empty event

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Date:   Mon, 27 Sep 2021 13:47:43 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Johannes Lundberg <jlundberg@...w.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        David Ahern <dsahern@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Florian Westphal <fw@...len.de>,
        Alexander Aring <aahringo@...hat.com>,
        Tonghao Zhang <xiangxia.m.yue@...il.com>,
        Yangbo Lu <yangbo.lu@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        netdev <netdev@...r.kernel.org>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH] fs: eventpoll: add empty event

On Mon, Sep 27, 2021 at 1:30 PM Johannes Lundberg <jlundberg@...w.com> wrote:
>
> The EPOLLEMPTY event will trigger when the TCP write buffer becomes
> empty, i.e., when all outgoing data have been ACKed.
>
> The need for this functionality comes from a business requirement
> of measuring with higher precision how much time is spent
> transmitting data to a client. For reference, similar functionality
> was previously added to FreeBSD as the kqueue event EVFILT_EMPTY.


Adding yet another indirect call [1] in TCP fast path, for something
(measuring with higher precision..)
which is already implemented differently in TCP stack [2] is not desirable.

Our timestamping infrastructure should be ported to FreeBSD instead :)

[1] CONFIG_RETPOLINE=y

[2] Refs :
   commit e1c8a607b28190cd09a271508aa3025d3c2f312e
      net-timestamp: ACK timestamp for bytestreams
    tools/testing/selftests/net/txtimestamp.c