netdev - Re: [net-next PATCH v2 2/2] e1000: bundle xdp xmit routines

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Date:   Fri, 9 Sep 2016 18:12:40 -0700
From:   John Fastabend <john.fastabend@...il.com>
To:     Tom Herbert <tom@...bertland.com>
Cc:     Brenden Blanco <bblanco@...mgrid.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Cong Wang <xiyou.wangcong@...il.com>,
        intel-wired-lan <intel-wired-lan@...ts.osuosl.org>,
        u9012063@...il.com,
        Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [net-next PATCH v2 2/2] e1000: bundle xdp xmit routines

On 16-09-09 06:04 PM, Tom Herbert wrote:
> On Fri, Sep 9, 2016 at 5:01 PM, John Fastabend <john.fastabend@...il.com> wrote:
>> On 16-09-09 04:44 PM, Tom Herbert wrote:
>>> On Fri, Sep 9, 2016 at 2:29 PM, John Fastabend <john.fastabend@...il.com> wrote:
>>>> e1000 supports a single TX queue so it is being shared with the stack
>>>> when XDP runs XDP_TX action. This requires taking the xmit lock to
>>>> ensure we don't corrupt the tx ring. To avoid taking and dropping the
>>>> lock per packet this patch adds a bundling implementation to submit
>>>> a bundle of packets to the xmit routine.
>>>>
>>>> I tested this patch running e1000 in a VM using KVM over a tap
>>>> device using pktgen to generate traffic along with 'ping -f -l 100'.
>>>>
>>> Hi John,
>>>
>>> How does this interact with BQL on e1000?
>>>
>>> Tom
>>>
>>
>> Let me check if I have the API correct. When we enqueue a packet to
>> be sent we must issue a netdev_sent_queue() call and then on actual
>> transmission issue a netdev_completed_queue().
>>
>> The patch attached here missed a few things though.
>>
>> But it looks like I just need to call netdev_sent_queue() from the
>> e1000_xmit_raw_frame() routine and then let the tx completion logic
>> kick in which will call netdev_completed_queue() correctly.
>>
>> I'll need to add a check for the queue state as well. So if I do these
>> three things,
>>
>>         check __QUEUE_STATE_XOFF before sending
>>         netdev_sent_queue() -> on XDP_TX
>>         netdev_completed_queue()
>>
>> It should work agree? Now should we do this even when XDP owns the
>> queue? Or is this purely an issue with sharing the queue between
>> XDP and stack.
>>
> But what is the action for XDP_TX if the queue is stopped? There is no
> qdisc to back pressure in the XDP path. Would we just start dropping
> packets then?

Yep that is what the patch does if there is any sort of error packets
get dropped on the floor. I don't think there is anything else that
can be done.

> 
> Tom
> 
>> .John
>>