Date:   Tue, 29 Aug 2017 09:23:49 -0700
From:   Alexander Duyck <alexander.duyck@...il.com>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     Andy Gospodarek <andy@...yhouse.net>,
        Michael Chan <michael.chan@...adcom.com>,
        John Fastabend <john.fastabend@...il.com>,
        "Duyck, Alexander H" <alexander.h.duyck@...el.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>,
        "borkmann@...earbox.net" <borkmann@...earbox.net>
Subject: Re: XDP redirect measurements, gotchas and tracepoints

On Tue, Aug 29, 2017 at 6:26 AM, Jesper Dangaard Brouer
<brouer@...hat.com> wrote:
>
> On Mon, 28 Aug 2017 09:11:25 -0700 Alexander Duyck <alexander.duyck@...il.com> wrote:
>
>> My advice would be to not over complicate this. My big concern with
>> all this buffer recycling is what happens the first time somebody
>> introduces something like mirroring? Are you going to copy the data to
>> a new page which would be quite expensive or just have to introduce
>> reference counts? You are going to have to deal with stuff like
>> reference counts eventually so you might as well bite that bullet now.
>> My advice would be to not bother with optimizing for performance right
>> now and instead focus on just getting functionality. The approach we
>> took in ixgbe for the transmit path should work for almost any other
>> driver since all you are looking at is having to free the page
>> reference which takes care of reference counting already.
>
> This return API is not about optimizing performance right now.  It is
> actually about allowing us to change the underlying memory model per RX
> queue for XDP.

 I would disagree. To me this is a obvious case of premature optimization.

> If a RX-ring is use for both SKBs and XDP, then the refcnt model is
> still enforced.  Although a driver using the 1-packet-per-page model,
> should be able to reuse refcnt==1 pages when returned from XDP.

Isn't this the case for all Rx on XDP enabled rings. Last I knew there
was an option to pass packets up via an SKB if XDP_PASS is returned.
Are you saying we need to do a special allocation path if an XDP
program doesn't make use of XDP_PASS?

> If a RX-ring is _ONLY_ used for XDP, then the driver have freedom to
> implement another memory model, with the return-API.  We need to
> experiment with the most optimal memory model.  The 1-packet-per-page
> model is actually not the fastest, because of PCI-e bottlenecks.  With
> HW support for packing descriptors and packets over the PCI-e bus, much
> higher rates can be achieved.  Mellanox mlx5-Lx already have the needed HW
> support.  And companies like NetCope also have 100G HW that does
> similar tricks, and they even have a whitepaper[1][2] how they are
> faster than DPDK with their NDP (Netcope Data Plane) API.
>
> We do need the ability/flexibility to change the RX memory model, to
> take advantage of this new NIC hardware.

Looking over the white paper I see nothing that prevents us from using
the same memory model we do with the Intel NICs. If anything I think
the Intel drivers in "legacy-rx" mode could support something like
this now, even if the hardware doesn't simply because we can get away
with keeping the memory pseudo-pinned. My bigger concern is that we
keep coming back to this idea that we need to have the network stack
taking care of the 1 page per packet recycling when I really think it
has no business being there. We either need to look at implementing
this in the way we did in the Intel drivers where we use the reference
counts or implement our own memory handling API like SLUB or something
similar based on compound page destructors. I would much rather see us
focus on getting this going with an agnostic memory model where we
don't have to make the stack aware of where the memory came from or
where it has to be returned to.

> [1] https://www.netcope.com/en/resources/improving-dpdk-performance
> [2] https://www.netcope.com/en/company/press-center/press-releases/read-new-netcope-whitepaper-on-dpdk-acceleration

My only concern with something like this is the fact that it is
optimized for a setup where the data is left in place and nothing
extra is added. Trying to work with something like this gets more
expensive when you have to deal with the full stack as you have to
copy out the headers and still deal with all the skb metadata. I fully
agree with the basic premise that writing in large blocks provides
significant gains in throughput, specifically with small packets. The
only gotcha you would have to deal with is SKB allocation and data
copying overhead to make room and fill in metadata for the frame and
any extra headers needed.

- Alex

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