netdev - Re: [Lsf] [Lsf-pc] [LSF/MM TOPIC] Generic page-pool recycle facility?

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Date:	Tue, 12 Apr 2016 08:16:49 +0200
From:	Jesper Dangaard Brouer <brouer@...hat.com>
To:	Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:	"lsf@...ts.linux-foundation.org" <lsf@...ts.linux-foundation.org>,
	James Bottomley <James.Bottomley@...senPartnership.com>,
	Sagi Grimberg <sagi@...mberg.me>,
	Tom Herbert <tom@...bertland.com>,
	Brenden Blanco <bblanco@...mgrid.com>,
	Christoph Hellwig <hch@...radead.org>,
	linux-mm <linux-mm@...ck.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Bart Van Assche <bart.vanassche@...disk.com>,
	"lsf-pc@...ts.linux-foundation.org" 
	<lsf-pc@...ts.linux-foundation.org>, brouer@...hat.com
Subject: Re: [Lsf] [Lsf-pc] [LSF/MM TOPIC] Generic page-pool recycle
 facility?


On Mon, 11 Apr 2016 15:21:26 -0700
Alexei Starovoitov <alexei.starovoitov@...il.com> wrote:

> On Mon, Apr 11, 2016 at 11:41:57PM +0200, Jesper Dangaard Brouer wrote:
> > 
> > On Sun, 10 Apr 2016 21:45:47 +0300 Sagi Grimberg <sagi@...mberg.me> wrote:
> >   
[...]
> > > 
> > > If we go down this road how about also attaching some driver opaques
> > > to the page sets?  
> > 
> > That was the ultimate plan... to leave some opaques bytes left in the
> > page struct that drivers could use.
> > 
> > In struct page I would need a pointer back to my page_pool struct and a
> > page flag.  Then, I would need room to store the dma_unmap address.
> > (And then some of the usual fields are still needed, like the refcnt,
> > and reusing some of the list constructs).  And a zero-copy cross-domain
> > id.  
> 
> I don't think we need to add anything to struct page.
> This is supposed to be small cache of dma_mapped pages with lockless access.
> It can be implemented as an array or link list where every element
> is dma_addr and pointer to page. If it is full, dma_unmap_page+put_page to
> send it to back to page allocator.

It sounds like the Intel drivers recycle facility, where they split the
page into two parts, and keep page in RX-ring, by swapping to other
half of page, if page_count(page) is <= 2.  Thus, they use the atomic
page ref count to synchronize on.

Thus, we end-up having two atomic operations per RX packet, on the page
refcnt.  Where DPDK have zero...

By fully taking over the page as an allocator, almost like slab. I can
optimize the common case (of the packet-page getting allocated and
free'ed on the same CPU), and remove these atomic operations.

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