Re: [dpdk-dev] [PATCH v4 1/3] ethdev: support API to set max LRO packet size

[Matan Azrad <matan@mellanox.com>]

From: Ananyev, Konstantin
> >
> > From: Ferruh Yigit
> > > On 11/8/2019 11:56 AM, Matan Azrad wrote:
> > > >
> > > >
> > > > From: Ferruh Yigit
> > > >> On 11/8/2019 10:10 AM, Matan Azrad wrote:
> > > >>>
> > > >>>
> > > >>> From: Ferruh Yigit
> > > >>>> On 11/8/2019 6:54 AM, Matan Azrad wrote:
> > > >>>>> Hi
> > > >>>>>
> > > >>>>> From: Ferruh Yigit
> > > >>>>>> On 11/7/2019 12:35 PM, Dekel Peled wrote:
> > > >>>>>>> @@ -1266,6 +1286,18 @@ struct rte_eth_dev *
> > > >>>>>>>
> > > >>>>>> 	RTE_ETHER_MAX_LEN;
> > > >>>>>>>  	}
> > > >>>>>>>
> > > >>>>>>> +	/*
> > > >>>>>>> +	 * If LRO is enabled, check that the maximum aggregated
> > > >> packet
> > > >>>>>>> +	 * size is supported by the configured device.
> > > >>>>>>> +	 */
> > > >>>>>>> +	if (dev_conf->rxmode.offloads &
> > > >> DEV_RX_OFFLOAD_TCP_LRO) {
> > > >>>>>>> +		ret = check_lro_pkt_size(
> > > >>>>>>> +				port_id, dev_conf-
> > > >>>>>>> rxmode.max_lro_pkt_size,
> > > >>>>>>> +				dev_info.max_lro_pkt_size);
> > > >>>>>>> +		if (ret != 0)
> > > >>>>>>> +			goto rollback;
> > > >>>>>>> +	}
> > > >>>>>>> +
> > > >>>>>>
> > > >>>>>> This check forces applications that enable LRO to provide
> > > >>>> 'max_lro_pkt_size'
> > > >>>>>> config value.
> > > >>>>>
> > > >>>>> Yes.(we can break an API, we noticed it)
> > > >>>>
> > > >>>> I am not talking about API/ABI breakage, that part is OK.
> > > >>>> With this check, if the application requested LRO offload but
> > > >>>> not provided 'max_lro_pkt_size' value, device configuration will fail.
> > > >>>>
> > > >>> Yes
> > > >>>> Can there be a case application is good with whatever the PMD
> > > >>>> can support as max?
> > > >>> Yes can be - you know, we can do everything we want but it is
> > > >>> better to be
> > > >> consistent:
> > > >>> Due to the fact of Max rx pkt len field is mandatory for JUMBO
> > > >>> offload, max
> > > >> lro pkt len should be mandatory for LRO offload.
> > > >>>
> > > >>> So your question is actually why both, non-lro packets and LRO
> > > >>> packets max
> > > >> size are mandatory...
> > > >>>
> > > >>>
> > > >>> I think it should be important values for net applications
> management.
> > > >>> Also good for mbuf size managements.
> > > >>>
> > > >>>>>
> > > >>>>>> - Why it is mandatory now, how it was working before if it is
> > > >>>>>> mandatory value?
> > > >>>>>
> > > >>>>> It is the same as max_rx_pkt_len which is mandatory for jumbo
> > > >>>>> frame
> > > >>>> offload.
> > > >>>>> So now, when the user configures a LRO offload he must to set
> > > >>>>> max lro pkt
> > > >>>> len.
> > > >>>>> We don't want to confuse the user here with the max rx pkt len
> > > >>>> configurations and behaviors, they should be with same logic.
> > > >>>>>
> > > >>>>> This parameter defines well the LRO behavior.
> > > >>>>> Before this, each PMD took its own interpretation to what
> > > >>>>> should be the
> > > >>>> maximum size for LRO aggregated packets.
> > > >>>>> Now, the user must say what is his intension, and the ethdev
> > > >>>>> can limit it
> > > >>>> according to the device capability.
> > > >>>>> By this way, also, the PMD can organize\optimize its data-path
> more.
> > > >>>>> Also, the application can create different mempools for LRO
> > > >>>>> queues to
> > > >>>> allow bigger packet receiving for LRO traffic.
> > > >>>>>
> > > >>>>>> - What happens if PMD doesn't provide 'max_lro_pkt_size', so
> > > >>>>>> it is
> > > '0'?
> > > >>>>> Yes, you can see the feature description Dekel added.
> > > >>>>> This patch also updates all the PMDs support an LRO for non-0
> value.
> > > >>>>
> > > >>>> Of course I can see the updates Matan, my point is "What
> > > >>>> happens if PMD doesn't provide 'max_lro_pkt_size'",
> > > >>>> 1) There is no check for it right, so it is acceptable?
> > > >>>
> > > >>> There is check.
> > > >>> If the capability is 0, any non-zero configuration will fail.
> > > >>>
> > > >>>> 2) Are we making this filed mandatory to provide for PMDs, it
> > > >>>> is easy to make new fields mandatory for PMDs but is this
> > > >>>> really
> > > necessary?
> > > >>>
> > > >>> Yes, for consistence.
> > > >>>
> > > >>>>>
> > > >>>>> as same as max rx pkt len, no?
> > > >>>>>
> > > >>>>>> - What do you think setting 'max_lro_pkt_size' config value
> > > >>>>>> to what PMD provided if application doesn't provide it?
> > > >>>>> Same answers as above.
> > > >>>>>
> > > >>>>
> > > >>>> If application doesn't care the value, as it has been till now,
> > > >>>> and not provided explicit 'max_lro_pkt_size', why not ethdev
> > > >>>> level use the value provided by PMD instead of failing?
> > > >>>
> > > >>> Again, same question we can ask on max rx pkt len.
> > > >>>
> > > >>> Looks like the packet size is very important value which should
> > > >>> be set by
> > > >> the application.
> > > >>>
> > > >>> Previous applications have no option to configure it, so they
> > > >>> haven't
> > > >> configure it, (probably cover it somehow) I think it is our miss
> > > >> to supply this info.
> > > >>>
> > > >>> Let's do it in same way as we do max rx pkt len (as this patch main
> idea).
> > > >>> Later, we can change both to other meaning.
> > > >>>
> > > >>
> > > >> I think it is not a good reason to introduce a new mandatory
> > > >> config option for application because of 'max_rx_pkt_len' does it.
> > > >
> > > > It is mandatory only if LRO offload is configured.
> > > >
> > > >> Will it work, if:
> > > >> - If application doesn't provide this value, use the PMD max
> > > >
> > > > May cause a problem if the mbuf size is not enough for the PMD
> maximum.
> > >
> > > OK, this is what I was missing, for this case I was thinking
> > > max_rx_pkt_len will be used but you already explained that
> > > application may want to use different mempools for LRO queues.
> > >
> > So , are you agree with the idea?
> >
> > > For this case shouldn't PMDs take the 'rxmode.max_lro_pkt_size' into
> > > account and program the device accordingly (of course in LRO enabled
> > > case) ?
> > > This part seems missing and should be highlighted to other PMD
> maintainers.
> >
> >
> > Yes, you are right.
> > PMDs must limit the LRO aggregated packet according to the new field,
> > And it probably very hard for the patch introducer to understand how to do
> it for each PMD.
> >
> > I think each new configuration requires other maintainers\developers
> > to adjust their own PMD code to the new configuration and it should be
> done in limited time.
> >
> > My suggestion here:
> > 1. To reserve the info field and the configuration field for rc2.(if
> > it is critical not to break ABI for rc3) 2. To merge the ethdev patch in the
> start of rc3.
> > 3. Request each relevant PMD to adjust its PMD to the new configuration
> for the end of rc3.
> > 	Note: this should be small change and only for ~5 PMDs:
> > 		a. Introduce the info field according to the device ability.
> > 		b. For each LRO queue:
> > 			Use the LRO max size configuration instead of the
> current max rx pkt len configuration(looks like small condition).
> 
> That's definitely looks like a significant behavior change for existing apps and
> PMDs, and I wonder what for?

There was a miss in configuration:

It doesn't make sense to limit non-lro queues with the same packets length of lro queues:
	Naturally, LRO packets are bigger significantly(because of the HW aggregation), hence,
	the user may use bigger mbufs for the LRO packets, so potentially, it is better to separate mempool, one for the LRO queues with big mbufs and the second for the non-LRO queues with smaller mbufs (to optimize the memory usage).
	Since the user may want tail-room in the LRO mbuf  it may limit the LRO packet size to smaller number than the mbuf (- HEADROOM) and for this reason as same as the usage of the regular field  (max_rx_pkt_len) a new field should be set for LRO queues.

> Why we can't keep max_rx_pkt_len semantics as it is right now, and just add
> an optional ability to limit max size of LRO aggregations?

What is the semantic of max_rx_pkt_len regards LRO packets? It is not clear from the documentation.
 
So this patch defines it well:
Non-LRO queues should be limited to max_rx_pkt_len.
LRO queues should be limited to max_lro_pkt_len.

The consistence in the ways of the configuration for RX packet length should be the same.
max_rx_pkt_len is mandatory for JUMBO offload => max_lro_pkt_len is mandatory for LRO offload.


Current applications uses LRO just need to configure the field same as current max_rx_pkt_len if they want to stay with the same behavior - really not a big change.
If the application want to improve their memory usage as I said above, the new fields allow it as well. 

> > What do you think?
> >
> >
> >