Re: [dpdk-dev] [PATCH 3/3] net/bonding: start ethdev prior to setting 8023ad flow

From: Jan Viktorin <hidden>
Date: 2021-07-13 11:06:13

On Tue, 13 Jul 2021 12:26:35 +0300
Andrew Rybchenko [off-list ref] wrote:

On 7/13/21 11:18 AM, Havlík Martin wrote:

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Dne 2021-07-12 15:07, Ori Kam napsal:

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Hi Jan,

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-----Original Message-----
From: Jan Viktorin <redacted>
Sent: Monday, July 12, 2021 12:46 AM

On Sun, 11 Jul 2021 08:49:18 +0000
Ori Kam [off-list ref] wrote:

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Hi Jan,

Hi Ori,

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-----Original Message-----
From: dev <redacted> On Behalf Of Jan Viktorin
Sent: Wednesday, July 7, 2021 6:54 PM

On Sun, 4 Jul 2021 15:18:01 +0000
Matan Azrad [off-list ref] wrote:

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From: Havlík Martin

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Dne 2021-06-23 09:04, Min Hu (Connor) napsal:

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在 2021/6/22 17:25, Martin Havlik 写道:

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When dedicated queues are enabled, mlx5 PMD fails to
install RTE Flows if the underlying ethdev is not
started: bond_ethdev_8023ad_flow_set(267) -

bond_ethdev_8023ad_flow_set:

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port

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not started (slave_port=0 queue_id=1)

Why mlx5 PMD doing flow create relys on port started ?
I noticed other PMDs did not has that reliance.

After looking into it, I really can't answer this mlx5
centered question. Closest related info we found so far
is the 5th point in
https://doc.dpdk.org/guides/prog_guide/rte_flow.html#caveats
but it only specifies it's the application's
responsibility and that flow rules are assumed invalid
after port stop/close/restart but doesn't say anything
about <stop - flow rule create - start> vs <stop - start
- flow rule create> where the former is the point of
failure on mlx5. I'm addressing the maintainers for mlx5,
who might know a bit more on the topic.

Hello Matan,

quoted

From rte_ethdev.h

* Please note that some configuration is not stored between
calls to
 * rte_eth_dev_stop()/rte_eth_dev_start(). The following
configuration will
 * be retained:
 *
 *     - MTU
 *     - flow control settings
 *     - receive mode configuration (promiscuous mode,

all-multicast

quoted

mode,
 *       hardware checksum mode, RSS/VMDQ settings etc.)
 *     - VLAN filtering configuration
 *     - default MAC address
 *     - MAC addresses supplied to MAC address array
 *     - flow director filtering mode (but not filtering
rules)
 *     - NIC queue statistics mappings

just after this section, you can find the following statement:

 * Any other configuration will not be stored and will need
to be
re-entered
 * before a call to rte_eth_dev_start().

It is not very clear how is this exactly related to flows
(and this applies for all the quoted section, I think) but at
least it can

be used as a
counter argument.

quoted

 

I agree the doc is not clear, as I see it flows are not part of
configuration, at least not when we are talking about rte_flow.

Agree.

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Mlx5 assumes flows are allowed to be configured only after
rte_eth_dev_start(). Before start \ after stop - no flow is
valid anymore.

I believe that this discussion is not about validity of
flows. Let the flows be invalid after calling to
rte_eth_dev_stop(). This is OK, flows must be recreated and
the bonding driver works this way. But why not *before
start*?

Think about it this way by changing the configuration you may
create invalid flows, for example, you can only change the
number of queues after port stop, so if you create a flow with
jump to queue 3 and then you remove queue 3 then, the flow that
is cached is not valid anymore. This goes for other
configuration that may affect the validity of a

flow.

This is a valid argument, of course. The thing is whether it is a
responsibility
of the PMD to take care of those corner cases or if this is up to
the application developer. If we respect the fact that calling to
stop invalidates all
flows then when you do:
 
 > port stop 0
 > flow create 0 ingress pattern ... / end actions queue 6 / end

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port config

rxq 3  > port start 0

it's clear that something is really wrong with the caller/user. I
would say that
this is an application bug. I would expect that you first
reconfigure port and
after that you modify flows. It seems quite logical and intuitive,
doesn't it?

I agree the use case I described is bad programming, but the same
logic can be
applied on stopping the port why flows should be deleted?

I just took into account what Matan has written. Certainly, it is not
necessary to delete flows on stop.

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And even more important on some HW and some flows can only be
inserted after
start since there is no real traffic gate, basically the start
means adding the flows
if a flow is in the HW it is working, adding a gate will result in
PPS degradation.

What do you mean by term "gate", here? Some FPGA logic? Or something
else? Why "adding a gate" would result in PPS degradation? Can you
make those ideas a bit clearer?

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Anyway, the PMD can possibly catch this rxq inconsistency but I
can imagine
that there are more complicated sitations than just changing
count of queues. Any idea for a more complex real case?

Some of the resource may be created only after start,
But I don't have a very good example now.

Please, try to find some reasonable example otherwise your argumentation
does not make much sense...

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Martin Havlik, can you please check how ixgbe and i40e behave in
the case
above with "rxq change after flow created"?

Both PMDs do not raise any errors in the mentioned case. The packets
that should go to the non-existent queue are dropped at some point
in the processing (from what I could see and understand).

Thanks for clarification. It would be nice if any other PMD can be
tested for this behaviour. Can sombody help us with other NICs?

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I'm glad for the discussion that is taking place but I feel we have
strayed from the original reason of it. That being the inability to
enable dedicated queues for bonding mode 8023AD on mlx5. We have
concluded that flow creation has to be done after port/device
start. So moving the function call to create the flow after device
start should solve the issue
(https://mails.dpdk.org/archives/dev/2021-June/212210.html). From my
testing, flow create after start is not a problem on Intel PMDs.

It would be good to hear what net/bonding maintainers think
about it. I see no conclusion.

If net/mlx5 behaviour is fixed, will it fix the initial
problem?

I believe that if mlx5 allows to create flows before calling to
rte_eth_dev_start() then the bonding would work as expected.

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I'm turning to you, Connor, as you have made the original question
on this. Is the patch I presented applicable?

Martin

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Does somebody know how other drivers behaves in this
situation? (We know and can check for Intel, there it does
not seem to be an issue.)

By the way, the mlx5 behaviour opens a (probably short) time
window between starting of a port and configuation of
filtering flows. You may want to start the port with
thousands of flows that apply just when the port starts (not
after, that's late). This may introduce glitches in filtering
and measuring of traffic (well, it is a

question how
serious issue could it be...).

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Agree but this is always true nothing is done is zero time and
even if it was the insertion is not in zero time, (assuming
that even if the flows are stored by the PMD until start they
still will not all be inserted in the same time)

Sorry, I probably do not understand well. Yes, creation of a flow
would never
be zero time, agree. But if I create flows before the port starts,
than I do not
have to care too much about how long the creation takes. Because
the card is
expected to be configured already before pushing the "start
button".

Maybe, you assume that the created flows would first be cached
inside the
PMD and when the port is finally started, it than transparently
write the
queues to the HW. But this is not what I was talking about, that's
maybe even
worse because you are hiding such behaviour from users...

(I do not know the exact mlx5 implementation details, so my
answer can miss something, sorry for that.)

You are correct this is HW/PMD implementation issue, in case of
Nvidia we must

By "must" you probably mean "would have to"? Is that correct?

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cache the flows and only insert it after start.
Since RTE flow is generic and HW is not generic sometimes the PMD
needs to
translate this difference and in this gray area there is a game
between best API best performance understanding what the user

Yes, as a former FPGA dev, I can understand your concerns. RTE Flow is
a difficult beast.

Jan

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really wants. This is why the doc should be very clear on what is
the expected. but this is also the reason why such document is
very hard to agree on.

Do I understand correctly that net/mlx5 maintainers will follow
up?

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Ori

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This matters for the bonding case as well, doesn't it?. It is
not desirable to accidently omit a packet that was received
by primary ingress logic instead of being redirected into the
dedicated queue.

Are there any chances that for mlx5 it would be possible to
insert flow rules before calling rte_eth_dev_start? Anyway,
the behaviour should be specified and documented in DPDK more
precisely to avoid such uncertainty in the future.

I agree the documentation should be fixed.

+1

Cc Thomas and Ferruh since ethdev documentation should be
clarified.

It looks like there is no consensus if the patch is a right
direction or wrong. For me it looks wrong taking all above
arguments in to account (mainly necessity to be able to
insert flows before pushing start button which enables the
traffic if HW supports it).

So, I'm applying first two patches and hold on this one.

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