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

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Message-ID: <20180308113722.GB28493@localhost>
Date:   Thu, 8 Mar 2018 12:37:22 +0100
From:   Miroslav Lichvar <mlichvar@...hat.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Jesus Sanchez-Palencia <jesus.sanchez-palencia@...el.com>,
        netdev@...r.kernel.org, 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
Subject: Re: [RFC v3 net-next 08/18] net: SO_TXTIME: Add clockid and
 drop_if_late params

On Wed, Mar 07, 2018 at 02:45:45PM -0800, Eric Dumazet wrote:
> On Wed, 2018-03-07 at 13:52 -0800, Jesus Sanchez-Palencia wrote:
> > > 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.

> Not convincing really :/
> 
> Next big feature needing one bit in sk_buff will add it, and add a
> 63bit hole.

Would it be possible to put the clockid in skb_shared_info? If that's
technically difficult or does not make sense, I'm ok with the clockid
being a socket option.

If a packet is sent immediately after changing the clockid via
setsockopt(), will it be still guaranteed that the packet is
restricted by the new id?

> 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'

Well, I'd not expect it to be a common use case, but a public NTP
server could be sending millions of packets per second in traffic
peaks (typically at *:00:00) over multiple interfaces.

-- 
Miroslav Lichvar