Re: XDP redirect measurements, gotchas and tracepoints

[John Fastabend <john.fastabend@gmail.com>]

On 08/25/2017 05:45 AM, Jesper Dangaard Brouer wrote:
> On Thu, 24 Aug 2017 20:36:28 -0700
> Michael Chan <michael.chan@broadcom.com> wrote:
> 
>> On Wed, Aug 23, 2017 at 1:29 AM, Jesper Dangaard Brouer
>> <brouer@redhat.com> wrote:
>>> On Tue, 22 Aug 2017 23:59:05 -0700
>>> Michael Chan <michael.chan@broadcom.com> wrote:
>>>  
>>>> On Tue, Aug 22, 2017 at 6:06 PM, Alexander Duyck
>>>> <alexander.duyck@gmail.com> wrote:  
>>>>> On Tue, Aug 22, 2017 at 1:04 PM, Michael Chan <michael.chan@broadcom.com> wrote:  
>>>>>>
>>>>>> Right, but it's conceivable to add an API to "return" the buffer to
>>>>>> the input device, right?  
>>>
>>> Yes, I would really like to see an API like this.
>>>  
>>>>>
>>>>> You could, it is just added complexity. "just free the buffer" in
>>>>> ixgbe usually just amounts to one atomic operation to decrement the
>>>>> total page count since page recycling is already implemented in the
>>>>> driver. You still would have to unmap the buffer regardless of if you
>>>>> were recycling it or not so all you would save is 1.000015259 atomic
>>>>> operations per packet. The fraction is because once every 64K uses we
>>>>> have to bulk update the count on the page.
>>>>>  
>>>>
>>>> If the buffer is returned to the input device, the input device can
>>>> keep the DMA mapping.  All it needs to do is to dma_sync it back to
>>>> the input device when the buffer is returned.  
>>>
>>> Yes, exactly, return to the input device. I really think we should
>>> work on a solution where we can keep the DMA mapping around.  We have
>>> an opportunity here to make ndo_xdp_xmit TX queues use a specialized
>>> page return call, to achieve this. (I imagine other arch's have a high
>>> DMA overhead than Intel)
>>>
>>> I'm not sure how the API should look.  The ixgbe recycle mechanism and
>>> splitting the page (into two packets) actually complicates things, and
>>> tie us into a page-refcnt based model.  We could get around this by
>>> each driver implementing a page-return-callback, that allow us to
>>> return the page to the input device?  Then, drivers implementing the
>>> 1-packet-per-page can simply check/read the page-refcnt, and if it is
>>> "1" DMA-sync and reuse it in the RX queue.
>>>  
>>
>> Yeah, based on Alex' description, it's not clear to me whether ixgbe
>> redirecting to a non-intel NIC or vice versa will actually work.  It
>> sounds like the output device has to make some assumptions about how
>> the page was allocated by the input device. 
> 
> Yes, exactly. We are tied into a page refcnt based scheme.
> 
> Besides the ixgbe page recycle scheme (which keeps the DMA RX-mapping)
> is also tied to the RX queue size, plus how fast the pages are returned.
> This makes it very hard to tune.  As I demonstrated, default ixgbe
> settings does not work well with XDP_REDIRECT.  I needed to increase
> TX-ring size, but it broke page recycling (dropping perf from 13Mpps to
> 10Mpps) so I also needed it increase RX-ring size.  But perf is best if
> RX-ring size is smaller, thus two contradicting tuning needed.
> 

The changes to decouple the ixgbe page recycle scheme (1pg per descriptor
split into two halves being the default) from the number of descriptors
doesn't look too bad IMO. It seems like it could be done by having some
extra pages allocated upfront and pulling those in when we need another
page.

This would be a nice iterative step we could take on the existing API.

> 
>> With buffer return API,
>> each driver can cleanly recycle or free its own buffers properly.
> 
> Yes, exactly. And RX-driver can implement a special memory model for
> this queue.  E.g. RX-driver can know this is a dedicated XDP RX-queue
> which is never used for SKBs, thus opening for new RX memory models.
> 
> Another advantage of a return API.  There is also an opportunity for
> avoiding the DMA map on TX. As we need to know the from-device.  Thus,
> we can add a DMA API, where we can query if the two devices uses the
> same DMA engine, and can reuse the same DMA address the RX-side already
> knows.
> 
> 
>> Let me discuss this further with Andy to see if we can come up with a
>> good scheme.
> 
> Sound good, looking forward to hear what you come-up with :-)
> 

I guess by this thread we will see a broadcom nic with redirect support
soon ;)