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

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Date:	Tue, 12 Apr 2016 10:20:12 -0700
From:	Alexei Starovoitov <alexei.starovoitov@...il.com>
To:	Jesper Dangaard Brouer <brouer@...hat.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>
Subject: Re: [Lsf] [Lsf-pc] [LSF/MM TOPIC] Generic page-pool recycle facility?

On Tue, Apr 12, 2016 at 08:16:49AM +0200, Jesper Dangaard Brouer wrote:
> 
> 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.

actually I'm proposing the opposite. one page = one packet.
I'm perfectly happy to waste half a page, since number of such pages is small
and performance matter more. Typical performance vs memory tradeoff.

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

the page recycling cache should have zero atomic ops per packet
otherwise it's non starter.

> 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.

slub is doing local cmpxchg. 40G networking cannot afford it per packet.
If it's amortized due to batching that will be ok.