[ovs-dev] [PATCH v2 6/8] netdev-dpdk: retry with queue action
Finn Christensen
fc at napatech.com
Fri Sep 15 09:15:52 UTC 2017
-----Original Message-----
From: Darrell Ball [mailto:dball at vmware.com]
Sent: 14. september 2017 21:35
To: Finn Christensen <fc at napatech.com>
Cc: dev at openvswitch.org
Subject: Re: [ovs-dev] [PATCH v2 6/8] netdev-dpdk: retry with queue
action
On 9/14/17, 1:14 AM, "Finn Christensen" <fc at napatech.com> wrote:
-----Original Message-----
From: ovs-dev-bounces at openvswitch.org [mailto:ovs-dev-
bounces at openvswitch.org] On Behalf Of Darrell Ball
Sent: 13. september 2017 18:18
To: Simon Horman <simon.horman at netronome.com>
Cc: dev at openvswitch.org
Subject: Re: [ovs-dev] [PATCH v2 6/8] netdev-dpdk: retry with queue
action
On 9/13/17, 2:57 AM, "Simon Horman"
<simon.horman at netronome.com> wrote:
On Tue, Sep 12, 2017 at 08:36:19AM +0000, Darrell Ball wrote:
>
> On 9/10/17, 11:14 PM, "ovs-dev-bounces at openvswitch.org on
behalf of Yuanhan Liu" <ovs-dev-bounces at openvswitch.org on behalf of
yliu at fridaylinux.org> wrote:
>
> On Fri, Sep 08, 2017 at 06:48:50PM +0200, Simon Horman wrote:
> > On Tue, Sep 05, 2017 at 05:22:59PM +0800, Yuanhan Liu wrote:
> > > From: Finn Christensen <fc at napatech.com>
> > >
> > > AFAIK, most (if not all) NICs (including Mellanox and Intel) do
not
> > > support a pure MARK action. It's required to be used together
with
> > > some other actions, like QUEUE.
> > >
> > > To workaround it, retry with a queue action when first try
failed.
> > >
> > > Moreover, some Intel's NIC (say XL710) needs the QUEUE
action set
> > > before the MARK action.
> > >
> > > Co-authored-by: Yuanhan Liu <yliu at fridaylinux.org>
> > > Signed-off-by: Finn Christensen <fc at napatech.com>
> > > Signed-off-by: Yuanhan Liu <yliu at fridaylinux.org>
> >
> > This feels a bit like the tail wagging the dog.
> > Is this the lowest level at which it makes sense to implement
> > this logic?
> >
> > If so then I wonder if some sort of probing would be in order
> > to avoid the cost of trying to add the flow twice to hardware
> > where the queue is required.
>
> Do you mean something like rte_flow capability query, like
whether
> a queue action is needed for a mark action? If so, yes, I do think
> we miss an interface like this.
>
> Note that even in this solution, the flow won't be created twice
> to the hardware, because the first try would be failed.
>
> [Darrell]
>
> Having an api to quey capability and avoid the first try to HW
would be nice, but there are dependencies
> on RTE, drivers etc and I don’t know definitive the api would
be.
>
> Also, as nics are added this capability needs to be done and
state needs to be kept in all cases.
>
> It is an enhancement and if done should be reliable.
Agreed. Though I was more thinking of probing the hardware rather
than
having a capability API - I expect this would remove several of the
dependencies you describe above.
[Darrell] I have been pondering the probing option as well. It is certainly a
valid option; we use it in other cases such as datapath probing. One of the
aspects that worries me here is maintaining the correct per interface
(essentially; although the attribute is per nic) state across various events
such as new ports being added, vswitchd restarts, races with flow creation.
It would be non-trivial I guess and probably appropriate for the next patch
series, if done.
In this case, we have what seems like a clear distinction b/w Napatech
which does not need the queue action workaround and everything else,
which does.
Besides the non-Napatech behavior, which is worrisome, maintaining the
difference for flow handling under the covers is concerning.
I wonder if we should be upfront as possible here and just have a dpdk
interface configuration – maybe something like “supports native HWOL
mark action” since the better behavior is the exception?
The interface config would be more robust than probing.
This would need documentation, of course.
[Finn]
I think the rte queue action should never be used here when using
partial HWOL. Not the way OVS handles multi queues today.
Maybe a "default queue" could be used in the dpdk PMDs when no
queue is specified in rte flow?
[Darrell] This is the Napatech case, where no queue action is needed; you
are suggesting programming a default queue in this case.
I don’t follow how this would be helpful/desired?
[Finn]
I was trying to make my view on this, not particularly arguing for the Napatech
Case.
Here is what I was thinking:
Taking the case of this partial HWOL, then we are trying to offload the flow
classification to HW, like "pre-classify and mark". Then this mark is used to
accelerate OVS while finding the actions to execute. Since we do leave all
processing of the actions to OVS, there is no way for the partial HWOL to
know, at rte flow creation time, where to send the pre-classified packets
(which is strictly needed when seen from a DPDK rte flow point of view).
When multiple queues are specified in OVS, a hash splitting mechanism
is used in the nic to support RSS. Then the nic is responsible for selecting the
right queue according to the configured algorithm for RSS.
OVS only needs to know how many queues to service per port. No knowledge
about the association b/w flow <-> rxq is used in OVS today.
When looking at full HWOL, all flow actions will have to be interpreted, supported
and programmed to NIC using rte flow - send directly to target port and we
do not have this issue.
Have I missed something?
Essentially, this is a mismatch between the rte flow impl functionality in
PMD and the needs in OVS for flow classification, in a partial HWOL setup.
[Darrell] Maybe this has been explored, but were additional workaround
actions, like RTE_FLOW_ACTION_TYPE_RSS considered ?
I guess we could make this essentially a NOP, but if mark action
could ride along with it, then that would be good.
[Finn]
No. We have not explored that.
I'm not sure I understand how you would use RTE_FLOW_ACTION_TYPE_RSS
In this situation.
However, we would not mind Darells proposal, it makes sense since
most nic PMDs unfortunately needs this and it is only relevant for partial
HWOL. We are mostly concerned with full HWOL and therefore see partial
HWOL as a failover when full HWOL could not be handled in nic, if enabled.
I think anyways we need documentation describing the difference b/w
nics in the dpdk documentation (howto part).
Assuming no such enhancement is appropriate at this time I would
still like to ask if this is the best place for this hardware-specific code?
[Darrell]
For OVS, the netdev-dpdk layer is the lowest layer.
This kind of workaround is hard to hide, since we are messing with the
rxq, so I think OVS needs to know that it is in effect anyways. An
alternative is to supply a mark and an ‘optional queue’ and let the driver
decide if the queue is needed and report back whether it was. This would
be hard to do across various drivers. Supporting in the rte layer would
require both rte and driver support, so even more support.
> A separate comment is we need to document which nics need
the queue action.
>
> Also, I think we should check errno in the present code.
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