Re: XDP redirect measurements, gotchas and tracepoints

On Thu, 24 Aug 2017 20:36:28 -0700
Michael Chan [off-list ref] wrote:

On Wed, Aug 23, 2017 at 1:29 AM, Jesper Dangaard Brouer
[off-list ref] wrote:

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On Tue, 22 Aug 2017 23:59:05 -0700
Michael Chan [off-list ref] wrote:
 

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On Tue, Aug 22, 2017 at 6:06 PM, Alexander Duyck
[off-list ref] wrote:  

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On Tue, Aug 22, 2017 at 1:04 PM, Michael Chan [off-list ref] wrote:  

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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.
 

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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.

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 :-)

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer