Date:   Wed, 11 Apr 2018 13:31:31 -0700
From:   Ivan Briano <ivan.briano@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Jesus Sanchez-Palencia <jesus.sanchez-palencia@...el.com>
Cc:     netdev@...r.kernel.org, jhs@...atatu.com, xiyou.wangcong@...il.com,
        jiri@...nulli.us, vinicius.gomes@...el.com,
        richardcochran@...il.com, anna-maria@...utronix.de,
        henrik@...tad.us, John Stultz <john.stultz@...aro.org>,
        levi.pearson@...man.com, edumazet@...gle.com, willemb@...gle.com,
        mlichvar@...hat.com
Subject: Re: [RFC v3 net-next 13/18] net/sched: Introduce the TBS Qdisc



On 04/11/2018 01:16 PM, Thomas Gleixner wrote:
> On Tue, 10 Apr 2018, Jesus Sanchez-Palencia wrote:
>>>> This will be provided by tbs if the socket which is transmitting packets is
>>>> configured for deadline mode.
>>>
>>> You don't want the socket to decide that. The qdisc into which a socket
>>> feeds defines the mode and the qdisc rejects requests with the wrong mode.
>>>
>>> Making a qdisc doing both and let the user decide what he wants it to be is
>>> not really going to fly. Especially if you have different users which want
>>> a different mode. It's clearly distinct functionality.
>>
>>
>> Ok, so just to make sure I got this right, are you suggesting that both the
>> 'tbs' qdisc *and* the socket (i.e. through SO_TXTIME) should have a config
>> parameter for specifying the txtime mode? This way if there is a mismatch,
>> packets from that socket are rejected by the qdisc.
> 
> Correct. The same is true if you try to set SO_TXTIME for something which
> is just routing regular traffic.
> 
>> (...)
>>>
>>>> Another question for this mode (but perhaps that applies to both modes) is, what
>>>> if the qdisc misses the deadline for *any* reason? I'm assuming it should drop
>>>> the packet during dequeue.
>>>
>>> There the question is how user space is notified about that issue. The
>>> application which queued the packet on time does rightfully assume that
>>> it's going to be on the wire on time.
>>>
>>> This is a violation of the overall scheduling plan, so you need to have
>>> a sane design to handle that.
>>
>> In addition to the qdisc stats, we could look into using the socket's error
>> queue to notify the application about that.
> 
> Makes sense.
>  
>>>> Putting it all together, we end up with:
>>>>
>>>> 1) a new txtime aware qdisc, tbs, to be used per queue. Its cli will look like:
>>>> $ tc qdisc add (...) tbs clockid CLOCK_REALTIME delta 150000 offload sorting
>>>
>>> Why CLOCK_REALTIME? The only interesting time in a TSN network is
>>> CLOCK_TAI, really.
>>
>> REALTIME was just an example here to show that the qdisc has to be configured
>> with a clockid parameter. Are you suggesting that instead both of the new qdiscs
>> (i.e. tbs and taprio) should always be using CLOCK_TAI implicitly?
> 
> I think so. It's _the_ network time on which everything is based on.
> 
>>>> 2) a new cmsg-interface for setting a per-packet timestamp that will be used
>>>> either as a txtime or as deadline by tbs (and further the NIC driver for the
>>>> offlaod case): SCM_TXTIME.
>>>>
>>>> 3) a new socket option: SO_TXTIME. It will be used to enable the feature for a
>>>> socket, and will have as parameters a clockid and a txtime mode (deadline or
>>>> explicit), that defines the semantics of the timestamp set on packets using
>>>> SCM_TXTIME.
>>>>
>>>> 4) a new #define DYNAMIC_CLOCKID 15 added to include/uapi/linux/time.h .
>>>
>>> Can you remind me why we would need that?
>>
>> So there is a "clockid" that can be used for the full hw offload modes. On this
>> case, the txtimes are in reference to the NIC's PTP clock, and, as discussed, we
>> can't just use a clockid that was computed from the fd pointing to /dev/ptpX .
> 
> And the NICs PTP clock is CLOCK_TAI, so there should be no reason to have
> yet another clock, right?
> 

Most likely, though you can technically have a different time domain
that is not based on TAI.

> Thanks,
> 
> 	tglx
> 

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