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

Ananyev, Konstantin konstantin.ananyev at intel.com
Thu Aug 8 12:48:35 CEST 2019


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.
> 
> 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.

Well, then I think you trying to introduce a new requirement for PMD.
Right now, as I can see it is optional, and supposed to be set only
when PMD RX path updates tso_segsz.

/**
 * When packets are coalesced by a hardware or virtual driver, this flag
 * can be set in the RX mbuf, meaning that the m->tso_segsz field is
 * valid and is set to the segment size of original packets.
 */
#define PKT_RX_LRO           (1ULL << 16)

> 
> > >
> > > 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?

I still in doubt is it really worth to support that feature at all...
Though if we'll decide to add it, then I think it needs to be 
a new capability DEV_RX_OFFLOAD_LRO_STAT (or so)
and probably new mbuf.ol_flag value for it.

Konstantin


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