netdev - Re: [RFC v3 net-next 08/18] net: SO_TXTIME: Add clockid and drop_if

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Date:   Wed, 07 Mar 2018 14:45:45 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Jesus Sanchez-Palencia <jesus.sanchez-palencia@...el.com>,
        netdev@...r.kernel.org
Cc:     jhs@...atatu.com, xiyou.wangcong@...il.com, jiri@...nulli.us,
        vinicius.gomes@...el.com, richardcochran@...il.com,
        intel-wired-lan@...ts.osuosl.org, anna-maria@...utronix.de,
        henrik@...tad.us, tglx@...utronix.de, john.stultz@...aro.org,
        levi.pearson@...man.com, edumazet@...gle.com, willemb@...gle.com,
        mlichvar@...hat.com
Subject: Re: [RFC v3 net-next 08/18] net: SO_TXTIME: Add clockid and
 drop_if_late params

On Wed, 2018-03-07 at 13:52 -0800, Jesus Sanchez-Palencia wrote:
> Hi,
...
> I should have mentioned on the commit msg, but the tc_drop_if_late is
> actually
> filling a 1 bit hole that was already there.
> 
> 
> > 
> > Do we really need 32 bits for a clockid_t ?
> 
> There is a 2 bytes hole just after tc_index, so a u16 clockid would
> fit
> perfectly without increasing the skbuffs size / cachelines any
> further.
> 
> From Richard's reply, it seems safe to just change the definition
> here if we
> make it explicit on the SCM_CLOCKID documentation the caveat about
> the max
> possible fd count for dynamic clocks.
> 
> How does that sound?

Not convincing really :/

Next big feature needing one bit in sk_buff will add it, and add a
63bit hole.

Then next feature(s) will happily consume 'because there are holes
anyway'.

Then at some point we will cross cache line boundary and performance
will take a 10 % hit.

It is a never ending trend.

If you really need 33 bits, then maybe we'll ask you to guard the new
bits with some #if IS_ENABLED(CONFIG_...) so that we can opt-out.

Why do we _really_ need dynamic clocks being supported in core
networking stack, other than 'that is needed to send 2 packets per
second with precise departure time and arbitrary user defined clocks,
so lets do that, and do not care of the other 10,000,000 packets we
receive/send per second'

I have one patch (TXCS, something that I called XPS in the past)
implementing the remote-freeing of skbs that help workloads where skb
are produced on cpu A and consumed on cpu B,
using an additional 16bit field that I have not upstreamed yet (even if
Mellanox folks want that), simply because of this additional field...

Maybe I should eat this hole before you take it ?

No, we need to be extra careful.