Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Subject: Re: [PATCH] fs: eventpoll: add empty event
To:     Eric Dumazet <edumazet@google.com>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        "David S. Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>,
        Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
        David Ahern <dsahern@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Florian Westphal <fw@strlen.de>,
        Alexander Aring <aahringo@redhat.com>,
        Tonghao Zhang <xiangxia.m.yue@gmail.com>,
        Yangbo Lu <yangbo.lu@nxp.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        netdev <netdev@vger.kernel.org>,
        Willem de Bruijn <willemb@google.com>
References: <20210927202923.7360-1-jlundberg@llnw.com>
 <CANn89iJP7xpVnw6UnZwnixaAh=2+5f571CiqepYi2sy3-1MXmQ@mail.gmail.com>
From:   Johannes Lundberg <jlundberg@llnw.com>
Message-ID: <c675343d-a5bc-dce0-bcde-8a952682e698@llnw.com>
Date:   Mon, 27 Sep 2021 14:17:16 -0700
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101
 Thunderbird/78.13.0
MIME-Version: 1.0
In-Reply-To: <CANn89iJP7xpVnw6UnZwnixaAh=2+5f571CiqepYi2sy3-1MXmQ@mail.gmail.com>
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Transfer-Encoding: 7bit
Content-Language: en-US
Precedence: bulk


On 9/27/21 1:47 PM, Eric Dumazet wrote:
> On Mon, Sep 27, 2021 at 1:30 PM Johannes Lundberg <jlundberg@llnw.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.