Date:   Mon, 13 Nov 2017 16:13:50 -0800
From:   Alexander Duyck <alexander.duyck@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Michael Ma <make0818@...il.com>,
        Stephen Hemminger <stephen@...workplumber.org>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        jianjun.duan@...baba-inc.com, xiangning.yu@...baba-inc.com
Subject: Re: Per-CPU Queueing for QoS

On Mon, Nov 13, 2017 at 3:08 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Mon, 2017-11-13 at 14:47 -0800, Alexander Duyck wrote:
>> On Mon, Nov 13, 2017 at 10:17 AM, Michael Ma <make0818@...il.com> wrote:
>> > 2017-11-12 16:14 GMT-08:00 Stephen Hemminger <stephen@...workplumber.org>:
>> >> On Sun, 12 Nov 2017 13:43:13 -0800
>> >> Michael Ma <make0818@...il.com> wrote:
>> >>
>> >>> Any comments? We plan to implement this as a qdisc and appreciate any early feedback.
>> >>>
>> >>> Thanks,
>> >>> Michael
>> >>>
>> >>> > On Nov 9, 2017, at 5:20 PM, Michael Ma <make0818@...il.com> wrote:
>> >>> >
>> >>> > Currently txq/qdisc selection is based on flow hash so packets from
>> >>> > the same flow will follow the order when they enter qdisc/txq, which
>> >>> > avoids out-of-order problem.
>> >>> >
>> >>> > To improve the concurrency of QoS algorithm we plan to have multiple
>> >>> > per-cpu queues for a single TC class and do busy polling from a
>> >>> > per-class thread to drain these queues. If we can do this frequently
>> >>> > enough the out-of-order situation in this polling thread should not be
>> >>> > that bad.
>> >>> >
>> >>> > To give more details - in the send path we introduce per-cpu per-class
>> >>> > queues so that packets from the same class and same core will be
>> >>> > enqueued to the same place. Then a per-class thread poll the queues
>> >>> > belonging to its class from all the cpus and aggregate them into
>> >>> > another per-class queue. This can effectively reduce contention but
>> >>> > inevitably introduces potential out-of-order issue.
>> >>> >
>> >>> > Any concern/suggestion for working towards this direction?
>> >>
>> >> In general, there is no meta design discussions in Linux development
>> >> Several developers have tried to do lockless
>> >> qdisc and similar things in the past.
>> >>
>> >> The devil is in the details, show us the code.
>> >
>> > Thanks for the response, Stephen. The code is fairly straightforward,
>> > we have a per-cpu per-class queue defined as this:
>> >
>> > struct bandwidth_group
>> > {
>> >     struct skb_list queues[MAX_CPU_COUNT];
>> >     struct skb_list drain;
>> > }
>> >
>> > "drain" queue is used to aggregate per-cpu queues belonging to the
>> > same class. In the enqueue function, we determine the cpu where the
>> > packet is processed and enqueue it to the corresponding per-cpu queue:
>> >
>> > int cpu;
>> > struct bandwidth_group *bwg = &bw_rx_groups[bwgid];
>> >
>> > cpu = get_cpu();
>> > skb_list_append(&bwg->queues[cpu], skb);
>> >
>> > Here we don't check the flow of the packet so if there is task
>> > migration or multiple threads sending packets through the same flow we
>> > theoretically can have packets enqueued to different queues and
>> > aggregated to the "drain" queue out of order.
>> >
>> > Also AFAIK there is no lockless htb-like qdisc implementation
>> > currently, however if there is already similar effort ongoing please
>> > let me know.
>>
>> The question I would have is how would this differ from using XPS w/
>> mqprio? Would this be a classful qdisc like HTB or a classless one
>> like mqprio?
>>
>> From what I can tell XPS would be able to get you your per-cpu
>> functionality, the benefit of it though would be that it would avoid
>> out-of-order issues for sockets originating on the local system. The
>> only thing I see as an issue right now is that the rate limiting with
>> mqprio is assumed to be handled via hardware due to mechanisms such as
>> DCB.
>
> I think one of the key point was in : " do busy polling from a per-class
> thread to drain these queues."
>
> I mentioned this idea in TX path of :
>
> https://netdevconf.org/2.1/slides/apr6/dumazet-BUSY-POLLING-Netdev-2.1.pdf

I think this is a bit different from that idea in that the busy
polling is transferring packets from a per-cpu qdisc to a per traffic
class queueing discipline. Basically it would be a busy_poll version
of qdisc_run that would be transferring packets from one qdisc onto
another instead of attempting to transmit them directly.

What I think is tripping me up is that I don't think this is even
meant to work with a multiqueue device. The description seems more
like a mqprio implementation feeding into a prio qdisc which is then
used for dequeue. To me it seems like this solution would be pulling
complexity off of the enqueue side and just adding it to the dequeue
side. The use of the "busy poll" is just to try to simplify things as
the agent would then be consolidating traffic from multiple per-cpu
queues onto one drain queue.

Structure wise this ends up looking not too different from mqprio, the
main difference though would be that this would be a classful qdisc
and that the virtual qdiscs we have for the traffic classes would need
to be replaced with actual qdiscs for handling the "drain" aspect of
this.

- Alex

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