[dpdk-dev] [PATCH 2/2] doc: announce new mbuf field for LRO

[Ananyev, Konstantin]

> 
> Hi Konstantin
> 
> From: Ananyev, Konstantin
> > Sent: Wednesday, August 7, 2019 1:18 PM
> > To: Matan Azrad <matan at mellanox.com>; dev at dpdk.org
> > Cc: Thomas Monjalon <thomas at monjalon.net>; Yigit, Ferruh
> > <ferruh.yigit at intel.com>; Andrew Rybchenko
> > <arybchenko at solarflare.com>; Olivier Matz <olivier.matz at 6wind.com>
> > Subject: RE: [PATCH 2/2] doc: announce new mbuf field for LRO
> >
> > Hi Matan,
> >
> > > > >
> > > > > The API breakage is because the ``tso_segsz`` field was documented
> > > > > for LRO.
> > > > >
> > > > > The ``tso_segsz`` field in mbuf indicates the size of each segment
> > > > > in the LRO packet in Rx path and should be provided by the LRO
> > > > > packet port.
> > > > >
> > > > > While the generic LRO packet may aggregate different segments
> > > > > sizes in one packet, it is impossible to expose this information
> > > > > for each segment by one field and it doesn't make sense to expose
> > > > > all the segments sizes in the mbuf.
> > > > >
> > > > > A new field may be added as union with the above field to expose
> > > > > the number of segments aggregated in the LRO packet.
> > > > >
> > > > > Signed-off-by: Matan Azrad <matan at mellanox.com>
> > > > > ---
> > > > >  doc/guides/rel_notes/deprecation.rst | 4 ++++
> > > > >  1 file changed, 4 insertions(+)
> > > > >
> > > > > diff --git a/doc/guides/rel_notes/deprecation.rst
> > > > > b/doc/guides/rel_notes/deprecation.rst
> > > > > index c0cd9bc..e826b69 100644
> > > > > --- a/doc/guides/rel_notes/deprecation.rst
> > > > > +++ b/doc/guides/rel_notes/deprecation.rst
> > > > > @@ -45,6 +45,10 @@ Deprecation Notices
> > > > >    - ``eal_parse_pci_DomBDF`` replaced by ``rte_pci_addr_parse``
> > > > >    - ``rte_eal_compare_pci_addr`` replaced by ``rte_pci_addr_cmp``
> > > > >
> > > > > +* mbuf: Remove ``tso_segsz`` mbuf field providing for LRO support.
> > > > > +Use union
> > > > > +  block for the field memory to be shared with a new field
> > > > > +``lro_segs_n``
> > > > > +  indicates the number of segments aggregated in the LRO packet.
> > > > > +
> > > >
> > > > Wonder how the upper layer will use that information (except for stats)?
> > > > Could you guys provide any examples?
> > >
> > > 1. Stats, allow to calc accurate PPS.
> > > 2. Supply accurate information unlike the seg size which cannot be
> > accurate.
> > > 2. Let the user all the information (segs num allow an average seg
> > > size calculation)
> >
> > So just for stats, right?
> 
> Stats it is one option.
> 
> The user configured LRO, means he wants X > 1 packets to be aggregated by the port.
> 
> Don't you think X is interesting for the user?
> 
> For example, maybe there is Y for the next calculation:
> 
> If average(X) < Y:
> 	Stop LRO - not very good for performance to aggregate small number of packets - stop LRO.
> 

Might be, but I think user can use other metrics (let say average aggregated packet size)
for that purpose. 

> 
> 
> > If so,  wouldn't it be more plausible to extend PMD itself to provide some
> > extra statistics?
> > Just a thought.
> 
> Yes, may be interesting but it can be redundant work when the user don't need it.
> 
> >
> > >
> > > > Also what PMD should do if HW does supports LRO, but doesn't to
> > > > information?
> > >
> > > If the PMD knows all the segments size he can calculate it, no?
> > > 0 means PMD doesn't support it.
> >
> > I mean HW/PMD might support LRO, but doesn't provide information about
> > number of coalesced segments.
> > What PMD should do in that case?
> 
> As I said, to set this field with 0 and set the PKT_RX_LRO flag in ol_flags.
> 0 in this case means support LRO but cannot supply the segments num.

Ok..., but then what for then to set PKT_RX_LRO at all?
>From PMD perspective it would be easier not to set that flag at all and not to touch tso_segsz.

> 
> Do you familiar with PMDs that supports LRO but cannot provide the segments num?
> If so, what do these PMDs can provide instead?

Yes, ixgbe PMD.
It does support TCP_LRO offload, and when enabled, does coalesce the packets,
but doesn't set PKT_RX_LRO and doesn't touch tso_segsz.

> 
> > Still  set DEV_RX_OFFLOAD_TCP_LRO as enabled RX offload, but don't set
> > PKT_RX_LRO flag in the RX-ed mbuf, even if it does contain coalesced
> > packets?
> 
> No, read above.
> 
> > As I understand that what happens now.
> > It is probably ok by me (as means no changes in ixgbe PMD)...
> > But wouldn't that mean no defined way for the user to determine will
> > HW/PMD provide that information or not?
> 
> Will compare to 0, see above.

I mean how the user will determine in advance would given PMD/HW provide that info in tso_segsz or not?
Wait for the first LRO packet? Something else?

> 
> 
> > Konstantin
> 
> 
> > > > >  * dpaa2: removal of ``rte_dpaa2_memsegs`` structure which has
> > > > > been
> > > > replaced
> > > > >    by a pa-va search library. This structure was earlier being used for
> > holding
> > > > >    memory segments used by dpaa2 driver for faster pa->va translation.
> > > > > This
> > > > > --
> > > > > 1.8.3.1