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

[Matan Azrad <matan@mellanox.com>]

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.
 


> 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.

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

> 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.


> 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