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

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Date:   Mon, 27 Sep 2021 14:17:16 -0700
From:   Johannes Lundberg <jlundberg@...w.com>
To:     Eric Dumazet <edumazet@...gle.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 9/27/21 1:47 PM, Eric Dumazet wrote:
> 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

Hi Eric

Thanks for the feedback! If there's a way to achieve the same thing with 
current Linux I'm all for it. I'll look into how to use timestamps for this.