netdev - Re: Per-CPU Queueing for QoS

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Date:   Mon, 13 Nov 2017 10:17:35 -0800
From:   Michael Ma <make0818@...il.com>
To:     Stephen Hemminger <stephen@...workplumber.org>
Cc:     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

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.