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

[Matan Azrad <matan@mellanox.com>]

From: Ananyev, Konstantin
> > Hi Konstantin
> >
> > From: Ananyev, Konstantin
> > > Sent: Wednesday, August 7, 2019 1:18 PM
> > > To: Matan Azrad <matan@mellanox.com>; dev@dpdk.org
> > > Cc: Thomas Monjalon <thomas@monjalon.net>; Yigit, Ferruh
> > > <ferruh.yigit@intel.com>; Andrew Rybchenko
> > > <arybchenko@solarflare.com>; Olivier Matz <olivier.matz@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@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.

Yes, but I think it is better to supply the segs number which is an accurate number instead of average size of segment.
Then, user can decide any calculation he prefers.

> 
> >
> >
> > > 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.

The user should know that LRO is working. LRO flag should be set in any case.

> >
> > 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.

I think it should be changed to set the flag.

> 
> >
> > > 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?

Or wait to for the first LRO packet, or we can add a new ethdev capability for it.

What do you think?

> 
> >
> >
> > > 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