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Date:   Thu, 10 Dec 2020 16:27:33 -0800
From:   Vinicius Costa Gomes <vinicius.gomes@...el.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        "Geva, Erez" <erez.geva.ext@...mens.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        Arnd Bergmann <arnd@...db.de>,
        Cong Wang <xiyou.wangcong@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        Jakub Kicinski <kuba@...nel.org>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Jiri Pirko <jiri@...nulli.us>,
        Alexei Starovoitov <ast@...nel.org>,
        Colin Ian King <colin.king@...onical.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Eyal Birger <eyal.birger@...il.com>,
        "Gustavo A . R . Silva" <gustavoars@...nel.org>,
        Jakub Sitnicki <jakub@...udflare.com>,
        John Ogness <john.ogness@...utronix.de>,
        Jon Rosen <jrosen@...co.com>,
        Kees Cook <keescook@...omium.org>,
        Marc Kleine-Budde <mkl@...gutronix.de>,
        Martin KaFai Lau <kafai@...com>,
        Matthieu Baerts <matthieu.baerts@...sares.net>,
        Andrei Vagin <avagin@...il.com>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        Ingo Molnar <mingo@...nel.org>,
        John Stultz <john.stultz@...aro.org>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Michal Kubecek <mkubecek@...e.cz>,
        Or Cohen <orcohen@...oaltonetworks.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Richard Cochran <richardcochran@...il.com>,
        Stefan Schmidt <stefan@...enfreihafen.org>,
        Xie He <xie.he.0141@...il.com>,
        Stephen Boyd <sboyd@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vladis Dronov <vdronov@...hat.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Frederic Weisbecker <frederic@...nel.org>,
        Vedang Patel <vedang.patel@...el.com>,
        "Sudler, Simon" <simon.sudler@...mens.com>,
        "Meisinger, Andreas" <andreas.meisinger@...mens.com>,
        "henning.schild@...mens.com" <henning.schild@...mens.com>,
        "jan.kiszka@...mens.com" <jan.kiszka@...mens.com>,
        "Zirkler, Andreas" <andreas.zirkler@...mens.com>
Subject: Re: [PATCH 1/3] Add TX sending hardware timestamp.

Willem de Bruijn <willemdebruijn.kernel@...il.com> writes:

>> > If I understand correctly, you are trying to achieve a single delivery time.
>> > The need for two separate timestamps passed along is only because the
>> > kernel is unable to do the time base conversion.
>>
>> Yes, a correct point.
>>
>> >
>> > Else, ETF could program the qdisc watchdog in system time and later,
>> > on dequeue, convert skb->tstamp to the h/w time base before
>> > passing it to the device.
>>
>> Or the skb->tstamp is HW time-stamp and the ETF convert it to system clock based.
>>
>> >
>> > It's still not entirely clear to me why the packet has to be held by
>> > ETF initially first, if it is held until delivery time by hardware
>> > later. But more on that below.
>>
>> Let plot a simple scenario.
>> App A send a packet with time-stamp 100.
>> After arrive a second packet from App B with time-stamp 90.
>> Without ETF, the second packet will have to wait till the interface hardware send the first packet on 100.
>> Making the second packet late by 10 + first packet send time.
>> Obviously other "normal" packets are send to the non-ETF queue, though they do not block ETF packets
>> The ETF delta is a barrier that the application have to send the packet before to ensure the packet do not tossed.
>
> Got it. The assumption here is that devices are FIFO. That is not
> necessarily the case, but I do not know whether it is in practice,
> e.g., on the i210.

On the i210 and i225, that's indeed the case, i.e. only the launch time
of the packet at the front of the queue is considered.

[...]

>> >>>>> It only requires that pacing qdiscs, both sch_etf and sch_fq,
>> >>>>> optionally skip queuing in their .enqueue callback and instead allow
>> >>>>> the skb to pass to the device driver as is, with skb->tstamp set. Only
>> >>>>> to devices that advertise support for h/w pacing offload.
>> >>>>>
>> >>>> I did not use "Fair Queue traffic policing".
>> >>>> As for ETF, it is all about ordering packets from different applications.
>> >>>> How can we achive it with skiping queuing?
>> >>>> Could you elaborate on this point?
>> >>>
>> >>> The qdisc can only defer pacing to hardware if hardware can ensure the
>> >>> same invariants on ordering, of course.
>> >>
>> >> Yes, this is why we suggest ETF order packets using the hardware time-stamp.
>> >> And pass the packet based on system time.
>> >> So ETF query the system clock only and not the PHC.
>> >
>> > On which note: with this patch set all applications have to agree to
>> > use h/w time base in etf_enqueue_timesortedlist. In practice that
>> > makes this h/w mode a qdisc used by a single process?
>>
>> A single process theoretically does not need ETF, just set the skb-> tstamp and use a pass through queue.
>> However the only way now to set TC_SETUP_QDISC_ETF in the driver is using ETF.
>
> Yes, and I'd like to eventually get rid of this constraint.
>

I'm interested in these kind of ideas :-)

What would be your end goal? Something like:
 - Any application is able to set SO_TXTIME;
 - We would have a best effort support for scheduling packets based on
 their transmission time enabled by default;
 - If the hardware supports, there would be a "offload" flag that could
 be enabled;

More or less this?


Cheers.
-- 
Vinicius

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