RE: [EXT] Re: [PATCH v1 2/4] mbuf: add second dynamic field member for VA only build

[Pavan Nikhilesh Bhagavatula <pbhagavatula@marvell.com>]

> -----Original Message-----
> From: Bruce Richardson <bruce.richardson@intel.com>
> Sent: Tuesday, August 30, 2022 2:06 PM
> To: Morten Brørup <mb@smartsharesystems.com>
> Cc: Shijith Thotton <sthotton@marvell.com>; dev@dpdk.org; Pavan
> Nikhilesh Bhagavatula <pbhagavatula@marvell.com>;
> Honnappa.Nagarahalli@arm.com; Jerin Jacob Kollanukkaran
> <jerinj@marvell.com>; olivier.matz@6wind.com;
> stephen@networkplumber.org; thomas@monjalon.net
> Subject: [EXT] Re: [PATCH v1 2/4] mbuf: add second dynamic field member
> for VA only build
> 
> External Email
> 
> ----------------------------------------------------------------------
> On Mon, Aug 29, 2022 at 08:32:20PM +0200, Morten Brørup wrote:
> >
> > > From: Shijith Thotton [mailto:sthotton@marvell.com]
> > > Sent: Monday, 29 August 2022 17.16
> > >
> > > mbuf physical address field is not used in builds which only uses VA.
> > > It is used to expand the dynamic field area.
> > >
> > > Signed-off-by: Shijith Thotton <sthotton@marvell.com>
> > > ---
> > >  lib/mbuf/rte_mbuf_core.h | 26 +++++++++++++++++---------
> > >  lib/mbuf/rte_mbuf_dyn.c  |  2 ++
> > >  2 files changed, 19 insertions(+), 9 deletions(-)
> > >
> > > diff --git a/lib/mbuf/rte_mbuf_core.h b/lib/mbuf/rte_mbuf_core.h
> > > index 81cb07c2e4..98ce62fd6a 100644
> > > --- a/lib/mbuf/rte_mbuf_core.h
> > > +++ b/lib/mbuf/rte_mbuf_core.h
> > > @@ -579,15 +579,23 @@ struct rte_mbuf {
> > >  	RTE_MARKER cacheline0;
> > >
> > >  	void *buf_addr;           /**< Virtual address of segment buffer.
> > > */
> > > -	/**
> > > -	 * Physical address of segment buffer.
> > > -	 * This field is invalid if the build is configured to use only
> > > -	 * virtual address as IOVA (i.e. RTE_IOVA_AS_VA is defined).
> > > -	 * Force alignment to 8-bytes, so as to ensure we have the exact
> > > -	 * same mbuf cacheline0 layout for 32-bit and 64-bit. This makes
> > > -	 * working on vector drivers easier.
> > > -	 */
> > > -	rte_iova_t buf_iova __rte_aligned(sizeof(rte_iova_t));
> > > +	RTE_STD_C11
> > > +	union {
> > > +		/**
> > > +		 * Physical address of segment buffer.
> > > +		 * This field is invalid if the build is configured to use
> > > only
> > > +		 * virtual address as IOVA (i.e. RTE_IOVA_AS_VA is
> > > defined).
> > > +		 * Force alignment to 8-bytes, so as to ensure we have the
> > > exact
> > > +		 * same mbuf cacheline0 layout for 32-bit and 64-bit. This
> > > makes
> > > +		 * working on vector drivers easier.
> > > +		 */
> > > +		rte_iova_t buf_iova __rte_aligned(sizeof(rte_iova_t));
> > > +		/**
> > > +		 * Reserved for dynamic field in builds where physical
> > > address
> > > +		 * field is invalid.
> > > +		 */
> > > +		uint64_t dynfield2;
> > > +	};
> > >
> > >  	/* next 8 bytes are initialised on RX descriptor rearm */
> > >  	RTE_MARKER64 rearm_data;
> >
> > I know that the intention here is to keep the rte_mbuf structure intact,
> which will certainly improve the probability of getting this patch series into
> DPDK.
> >
> > So, I will add a comment for the benefit of the other participants in the
> discussion:
> >
> > With this patch, and in RTE_IOVA_AS_VA mode, it becomes possible to
> move m->next into the first cache line, so rte_pktmbuf_prefree_seg() does
> not have to touch the second cache line, thus potentially improving
> performance by eliminating one cache miss per freed packet segment. (I also
> recall someone mentioning that some PMDs set m->next on RX... If that is
> the case, a cache miss per packet might also be avoidable in those PMDs.)
> >
> > Obviously, moving m->next to the first cache line is not related to this patch
> series, but would belong in a completely different patch.
> >
> 
> +1 to that, with the exception that if it is decided to move the next
> pointer rather than use this as dynamic space, I think it *should* be in
> this patch series, rather than mucking about with mbuf twice. :-)

+1 When RTE_IOVA_AS_VA is set we can set mbuf->next as the dynamic field and move it to mbuf->buf_iova.
mbuf->next write is one of the prominent hotspot in arm platforms.