Date:   Fri, 9 Jul 2021 11:41:15 +0200
From:   Yannick Vignon <yannick.vignon@....nxp.com>
To:     Vinicius Costa Gomes <vinicius.gomes@...el.com>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     Jamal Hadi Salim <jhs@...atatu.com>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Jiri Pirko <jiri@...nulli.us>,
        "David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
        Joakim Zhang <qiangqing.zhang@....com>,
        sebastien.laveze@....nxp.com,
        Yannick Vignon <yannick.vignon@....com>,
        Kurt Kanzenbach <kurt@...utronix.de>,
        Ivan Khoronzhuk <ivan.khoronzhuk@...aro.org>,
        Vladimir Oltean <olteanv@...il.com>,
        Vedang Patel <vedang.patel@...el.com>,
        Michael Walle <michael@...le.cc>
Subject: Re: [PATCH net v1] net: taprio offload: enforce qdisc to netdev queue
 mapping

On 5/18/2021 4:34 PM, Yannick Vignon wrote:
> On 5/18/2021 12:42 AM, Vinicius Costa Gomes wrote:
>> Yannick Vignon <yannick.vignon@....nxp.com> writes:
>>
>>> On 5/15/2021 1:47 AM, Vinicius Costa Gomes wrote:
>>>> Jakub Kicinski <kuba@...nel.org> writes:
>>>>
>>>>> On Fri, 14 May 2021 13:40:58 -0700 Vinicius Costa Gomes wrote:
>>>>>> Jakub Kicinski <kuba@...nel.org> writes:
>>>>>>> You haven't CCed anyone who worked on this Qdisc in the last 2 
>>>>>>> years :/
>>>>>>> CCing them now. Comments, anyone?
>>>>>>
>>>>>> I guess I should suggest myself as maintainer, to reduce chances 
>>>>>> of this
>>>>>> happening again.
>>>>>
>>>>> Yes, please.
>>>>>
>>>>>>> This looks like a very drastic change. Are you expecting the 
>>>>>>> qdisc will
>>>>>>> always be bypassed?
>>>>>>
>>>>>> Only when running in full offload mode it will be bypassed.
>>>>>>
>>>>>> And it's kind of by design, in offload mode, the idea was: 
>>>>>> configure the
>>>>>> netdev traffic class to queue mapping, send the schedule to the 
>>>>>> hardware
>>>>>> and stay out of the way.
>>>>>>
>>>>>> But as per Yannick's report, it seems that taprio doesn't stay enough
>>>>>> out of the yay.
>>>>>>
>>>>>>> After a 1 minute looks it seems like taprio is using device 
>>>>>>> queues in
>>>>>>> strict priority fashion. Maybe a different model is needed, but a 
>>>>>>> qdisc
>>>>>>> with:
>>>>>>>
>>>>>>> enqueue()
>>>>>>> {
>>>>>>>     WARN_ONCE(1)
>>>>>>> }
>>>>>>>
>>>>>>> really doesn't look right to me.
>>>>>>
>>>
>>> My idea was to follow the logic of the other qdiscs dealing with
>>> hardware multiqueue, namely mq and mqprio. Those do not have any
>>> enqueue/dequeue callbacks, but instead define an attach callback to map
>>> the child qdiscs to the HW queues. However, for taprio all those
>>> callbacks are already defined by the time we choose between software and
>>> full-offload, so the WARN_ONCE was more out of extra caution in case I
>>> missed something. If my understanding is correct however, it would
>>> probably make sense to put a BUG() instead, since those code paths
>>> should never trigger with this patch.
>>>
>>> OTOH what did bother me a bit is that because I needed an attach
>>> callback for the full-offload case, I ended up duplicating some code
>>> from qdisc_graft in the attach callback, so that the software case would
>>> continue behaving as is.
>>>
>>> Those complexities could be removed by pulling out the full-offload case
>>> into its own qdisc, but as I said it has other drawbacks.
>>>
>>>>>> This patch takes the "stay out of the way" to the extreme, I kind of
>>>>>> like it/I am not opposed to it, if I had this idea a couple of years
>>>>>> ago, perhaps I would have used this same approach.
>>>>>
>>>>> Sorry for my ignorance, but for TXTIME is the hardware capable of
>>>>> reordering or the user is supposed to know how to send packets?
>>>>
>>>> At least the hardware that I am familiar with doesn't reorder packets.
>>>>
>>>> For TXTIME, we have ETF (the qdisc) that re-order packets. The way
>>>> things work when taprio and ETF are used together is something like
>>>> this: taprio only has enough knowledge about TXTIME to drop packets 
>>>> that
>>>> would be transmitted outside their "transmission window" (e.g. for
>>>> traffic class 0 the transmission window is only for 10 to 50, the 
>>>> TXTIME
>>>> for a packet is 60, this packet is "invalid" and is dropped). And then
>>>> when the packet is enqueued to the "child" ETF, it's re-ordered and 
>>>> then
>>>> sent to the driver.
>>>>
>>>> And this is something that this patch breaks, the ability of dropping
>>>> those invalid packets (I really wouldn't like to do this verification
>>>> inside our drivers). Thanks for noticing this.
>>>>
>>>
>>> Hmm, indeed, I missed that check (we don't use ETF currently). I'm not
>>> sure of the best way forward, but here are a few thoughts:
>>> . The problem only arises for full-offload taprio, not for the software
>>> or TxTime-assisted cases.
>>> . I'm not sure mixing taprio(full-offload) with etf(no-offload) is very
>>> useful, at least with small gate intervals: it's likely you will miss
>>> your window when trying to send a packet at exactly the right time in
>>> software (I am usually testing taprio with a 2ms period and a 4µs
>>> interval for the RT stream).
>>> . That leaves the case of taprio(full-offload) with etf(offload). Right
>>> now with the current stmmac driver config, a packet whose tstamp is
>>> outside its gate interval will be sent on the next interval (and block
>>> the queue).
>>
>> This is the case that is a bit problematic with our hardware. (full 
>> taprio
>> offload + ETF offload).
> 
> Based on your previous comment to Michael, this is starting to look a 
> lot like a work-around for a hardware bug. Would moving it to the 
> corresponding driver really be out of the question?
> Note: there are currently only 4 drivers implementing ETF (including 2 
> from Intel), so validating their behavior with late packets would likely 
> be doable if needed.
> 
>>> . The stmmac hardware supports an expiryTime, currently unsupported in
>>> the stmmac driver, which I think could be used to drop packets whose
>>> tstamps are wrong (the packet would be dropped once the tstamp
>>> "expires"). We'd need to add an API for configuration though, and it
>>> should be noted that the stmmac config for this is global to the MAC,
>>> not per-queue (so a config through sch-etf would affect all queues).
>>> . In general using taprio(full-offload) with etf(offload) will incur a
>>> small latency penalty: you need to post the packet before the ETF qdisc
>>> wakes up (plus some margin), and the ETF qdisc must wake up before the
>>> tx stamp (plus some margin). If possible (number of streams/apps <
>>> number of hw queues), it would be better to just use
>>> taprio(full-offload) alone, since the app will need to post the packet
>>> before the gate opens (so plus one margin, not 2).
>>
>> It really depends on the workload, and how the schedule is organized,
>> but yeah, that might be possible (for some cases :-)).
>>
>>>
>>>
>>>>>
>>>>> My biggest problem with this patch is that unless the application is
>>>>> very careful that WARN_ON_ONCE(1) will trigger. E.g. if softirq is
>>>>> servicing the queue when the application sends - the qdisc will not
>>>>> be bypassed, right?
>>>
>>> See above, unless I'm mistaken the "root" qdisc is never
>>> enqueued/dequeued for multi-queue aware qdiscs.
>>>
>>
>> That's true, mq and mqprio don't have enqueue()/dequeue(), but I think
>> that's more a detail of their implementation than a rule (that no
>> multiqueue-aware root qdisc should implement enqueue()/dequeue()).
>>
>> That is, from my point of view, there's nothing wrong in having a root
>> qdisc that's also a shaper/scheduler.
>>
>>>>>> I am now thinking if this idea locks us out of anything.
>>>>>>
>>>>>> Anyway, a nicer alternative would exist if we had a way to tell 
>>>>>> the core
>>>>>> "this qdisc should be bypassed" (i.e. don't call enqueue()/dequeue())
>>>>>> after init() runs.
>>>>>
>>>
>>> Again, I don't think enqueue/dequeue are called unless the HW queues
>>> point to the root qdisc. But this does raise an interesting point: the
>>> "scheduling" issue I observed was on the dequeue side, when all the
>>> queues were dequeued within the RT process context. If we could point
>>> the enqueue side to the taprio qdisc and the dequeue side to the child
>>> qdiscs, that would probably work (but I fear that would be a significant
>>> change in the way the qdisc code works).
>>
>> I am wondering if there's a simpler solution, right now (as you pointed
>> out) taprio traverses all queues during dequeue(), that's the problem.
>>
>> What I am thinking is if doing something like:
>>
>>       sch->dev_queue - netdev_get_tx_queue(dev, 0);
>>
>> To get the queue "index" and then only dequeuing packets for that queue,
>> would solve the issue. (A bit ugly, I know).
>>
>> I just wanted to write the idea down to see if any one else could find
>> any big issues with it. I will try to play with it a bit, if no-one
>> beats me to it.
> 
> I looked into it this morning, but I'm not sure how that would work. 
> With the current code, when qdisc_run() is executed sch->dev_queue will 
> always be 0 (sch being the taprio qdisc), so you'd need my proposed 
> patch or something similar for sch to point to the child qdiscs, but at 
> that point you'd also be bypassing the taprio qdisc on enqueue.
> 
> If removing the TxTime check inside taprio_queue remains difficult, I am 
> wondering if keeping my patch with the change below would work (not 
> tested):
> ***********************************************************************
> @@ -4206,7 +4206,10 @@ static int __dev_queue_xmit(struct sk_buff *skb, 
> struct net_device *sb_dev)
>                  skb_dst_force(skb);
> 
>          txq = netdev_core_pick_tx(dev, skb, sb_dev);
> -       q = rcu_dereference_bh(txq->qdisc);
> +       if (dev->qdisc->enqueue)
> +               q = dev->qdisc;
> +       else
> +               q = rcu_dereference_bh(txq->qdisc);
> 
>          trace_net_dev_queue(skb);
>          if (q->enqueue) {
> ***********************************************************************
> One risk though for multiqueue-aware qdiscs that define an enqueue (only 
> taprio is in that case today) is that the child qdisc selected by 
> enqueue would not match the txq from netdev_core_pick_tx, so in the end 
> we would call qdisc_run on the wrong qdisc...
> 

I haven't had much time to work on this, but the "naive" approach I 
described just above didn't work as is.

More importantly, I thought this patch was still under discussion, yet 
it seems it got merged into netdev, and on May 13th, so even before this 
thread started...
Did I miss something in the submission process?

> 
>>>
>>>>> I don't think calling enqueue() and dequeue() is a problem. The 
>>>>> problem
>>>>> is that RT process does unrelated work.
>>>>
>>>> That is true. But this seems like a much bigger (or at least more
>>>> "core") issue.
>>>>
>>>>
>>>> Cheers,
>>>>
>>>
>>
>>
>> Cheers,
>>
> 

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