netdev - Re: [PATCH v3 net-next 08/14] mlx4: use order-0 pages for RX

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Date:   Tue, 14 Feb 2017 21:02:19 +0100
From:   Jesper Dangaard Brouer <brouer@...hat.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     Alexander Duyck <alexander.duyck@...il.com>,
        Tariq Toukan <ttoukan.linux@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Tariq Toukan <tariqt@...lanox.com>,
        Martin KaFai Lau <kafai@...com>,
        Saeed Mahameed <saeedm@...lanox.com>,
        Willem de Bruijn <willemb@...gle.com>,
        Brenden Blanco <bblanco@...mgrid.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Eric Dumazet <eric.dumazet@...il.com>,
        linux-mm <linux-mm@...ck.org>, brouer@...hat.com
Subject: Re: [PATCH v3 net-next 08/14] mlx4: use order-0 pages for RX

On Tue, 14 Feb 2017 11:02:01 -0800
Eric Dumazet <edumazet@...gle.com> wrote:

> On Tue, Feb 14, 2017 at 10:46 AM, Jesper Dangaard Brouer
> <brouer@...hat.com> wrote:
> >  
> 
> >
> > With this Intel driver page count based recycle approach, the recycle
> > size is tied to the size of the RX ring.  As Eric and Tariq discovered.
> > And for other performance reasons (memory footprint of walking RX ring
> > data-structures), don't want to increase the RX ring sizes.  Thus, it
> > create two opposite performance needs.  That is why I think a more
> > explicit approach with a pool is more attractive.
> >
> > How is this approach doing to work for XDP?
> > (XDP doesn't "share" the page, and in-general we don't want the extra
> > atomic.)
> >
> > We absolutely need recycling with XDP, when transmitting out another
> > device, and the other devices DMA-TX completion need some way of
> > returning this page.
> > What is basically needed is a standardized callback to allow the remote
> > driver to return the page to the originating driver.  As we don't have
> > a NDP for XDP-forward/transmit yet, we could pass this callback as a
> > parameter along with the packet-page to send?
> >
> >  
> 
> 
> mlx4 already has a cache for XDP.
> I believe I did not change this part, it still should work.
> 
> commit d576acf0a22890cf3f8f7a9b035f1558077f6770
> Author: Brenden Blanco <bblanco@...mgrid.com>
> Date:   Tue Jul 19 12:16:52 2016 -0700
> 
>     net/mlx4_en: add page recycle to prepare rx ring for tx support

This obviously does not work for the case I'm talking about
(transmitting out another device with XDP).

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