Re: [PATCH] fs: eventpoll: add empty event

[Johannes Lundberg <jlundberg@llnw.com>]

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.