netdev - Re: XDP redirect measurements, gotchas and tracepoints

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Date:   Thu, 24 Aug 2017 20:36:28 -0700
From:   Michael Chan <michael.chan@...adcom.com>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     Alexander Duyck <alexander.duyck@...il.com>,
        "Duyck, Alexander H" <alexander.h.duyck@...el.com>,
        "john.fastabend@...il.com" <john.fastabend@...il.com>,
        "pstaszewski@...are.pl" <pstaszewski@...are.pl>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "xdp-newbies@...r.kernel.org" <xdp-newbies@...r.kernel.org>,
        "andy@...yhouse.net" <andy@...yhouse.net>,
        "borkmann@...earbox.net" <borkmann@...earbox.net>
Subject: Re: XDP redirect measurements, gotchas and tracepoints

On Wed, Aug 23, 2017 at 1:29 AM, Jesper Dangaard Brouer
<brouer@...hat.com> wrote:
> On Tue, 22 Aug 2017 23:59:05 -0700
> Michael Chan <michael.chan@...adcom.com> wrote:
>
>> On Tue, Aug 22, 2017 at 6:06 PM, Alexander Duyck
>> <alexander.duyck@...il.com> wrote:
>> > On Tue, Aug 22, 2017 at 1:04 PM, Michael Chan <michael.chan@...adcom.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.  With buffer return API,
each driver can cleanly recycle or free its own buffers properly.

Let me discuss this further with Andy to see if we can come up with a
good scheme.