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

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Date:   Fri, 24 Feb 2017 10:42:15 +0100
From:   Jesper Dangaard Brouer <brouer@...hat.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Alexander Duyck <alexander.duyck@...il.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Alexander Duyck <alexander.h.duyck@...el.com>,
        "David S . Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Tariq Toukan <tariqt@...lanox.com>,
        Saeed Mahameed <saeedm@...lanox.com>,
        Willem de Bruijn <willemb@...gle.com>,
        Brenden Blanco <bblanco@...mgrid.com>,
        Alexei Starovoitov <ast@...nel.org>, brouer@...hat.com
Subject: Re: [PATCH v3 net-next 08/14] mlx4: use order-0 pages for RX

On Wed, 22 Feb 2017 18:06:58 -0800
Eric Dumazet <eric.dumazet@...il.com> wrote:

> On Wed, 2017-02-22 at 17:08 -0800, Alexander Duyck wrote:
> 
> > 
> > Right but you were talking about using both halves one after the
> > other.  If that occurs you have nothing left that you can reuse.  That
> > was what I was getting at.  If you use up both halves you end up
> > having to unmap the page.
> >   
> 
> You must have misunderstood me.

FYI, I also misunderstood you (Eric) to start with ;-)
 
> Once we use both halves of a page, we _keep_ the page, we do not unmap
> it.
> 
> We save the page pointer in a ring buffer of pages.
> Call it the 'quarantine'
> 
> When we _need_ to replenish the RX desc, we take a look at the oldest
> entry in the quarantine ring.
> 
> If page count is 1 (or pagecnt_bias if needed) -> we immediately reuse
> this saved page.
> 
> If not, _then_ we unmap and release the page.
> 
> Note that we would have received 4096 frames before looking at the page
> count, so there is high chance both halves were consumed.
> 
> To recap on x86 :
> 
> 2048 active pages would be visible by the device, because 4096 RX desc
> would contain dma addresses pointing to the 4096 halves.
> 
> And 2048 pages would be in the reserve.

I do like it, and it should work.  I like it because it solves my
concern, regarding being able to adjust the amount of
outstanding-frames independently of the RX ring size.


Do notice: driver developers have to use Alex'es new DMA API in-order
to get writable-pages, else this will violate the DMA API.  And XDP
requires writable pages.

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