Message-ID: <363f85d5-73bf-0b8e-dd60-5fc234d1d177@ovn.org>
Date:   Thu, 9 Feb 2023 01:09:21 +0100
From:   Ilya Maximets <i.maximets@....org>
To:     Marcelo Leitner <mleitner@...hat.com>,
        Paul Blakey <paulb@...dia.com>
Cc:     i.maximets@....org, netdev@...r.kernel.org,
        Saeed Mahameed <saeedm@...dia.com>,
        Paolo Abeni <pabeni@...hat.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Cong Wang <xiyou.wangcong@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Oz Shlomo <ozsh@...dia.com>, Jiri Pirko <jiri@...dia.com>,
        Roi Dayan <roid@...dia.com>, Vlad Buslov <vladbu@...dia.com>
Subject: Re: [PATCH net-next v8 0/7] net/sched: cls_api: Support hardware miss
 to tc action

On 2/8/23 19:01, Marcelo Leitner wrote:
> On Wed, Feb 08, 2023 at 10:41:39AM +0200, Paul Blakey wrote:
>>
>>
>> On 07/02/2023 07:03, Marcelo Leitner wrote:
>>> On Tue, Feb 07, 2023 at 01:20:55AM +0100, Ilya Maximets wrote:
>>>> On 2/6/23 18:14, Paul Blakey wrote:
>>>>>
>>>>>
>>>>> On 06/02/2023 14:34, Ilya Maximets wrote:
>>>>>> On 2/5/23 16:49, Paul Blakey wrote:
>>>>>>> Hi,
>>>>>>>
>>>>>>> This series adds support for hardware miss to instruct tc to continue execution
>>>>>>> in a specific tc action instance on a filter's action list. The mlx5 driver patch
>>>>>>> (besides the refactors) shows its usage instead of using just chain restore.
>>>>>>>
>>>>>>> Currently a filter's action list must be executed all together or
>>>>>>> not at all as driver are only able to tell tc to continue executing from a
>>>>>>> specific tc chain, and not a specific filter/action.
>>>>>>>
>>>>>>> This is troublesome with regards to action CT, where new connections should
>>>>>>> be sent to software (via tc chain restore), and established connections can
>>>>>>> be handled in hardware.
>>>>>>>
>>>>>>> Checking for new connections is done when executing the ct action in hardware
>>>>>>> (by checking the packet's tuple against known established tuples).
>>>>>>> But if there is a packet modification (pedit) action before action CT and the
>>>>>>> checked tuple is a new connection, hardware will need to revert the previous
>>>>>>> packet modifications before sending it back to software so it can
>>>>>>> re-match the same tc filter in software and re-execute its CT action.
>>>>>>>
>>>>>>> The following is an example configuration of stateless nat
>>>>>>> on mlx5 driver that isn't supported before this patchet:
>>>>>>>
>>>>>>>    #Setup corrosponding mlx5 VFs in namespaces
>>>>>>>    $ ip netns add ns0
>>>>>>>    $ ip netns add ns1
>>>>>>>    $ ip link set dev enp8s0f0v0 netns ns0
>>>>>>>    $ ip netns exec ns0 ifconfig enp8s0f0v0 1.1.1.1/24 up
>>>>>>>    $ ip link set dev enp8s0f0v1 netns ns1
>>>>>>>    $ ip netns exec ns1 ifconfig enp8s0f0v1 1.1.1.2/24 up
>>>>>>>
>>>>>>>    #Setup tc arp and ct rules on mxl5 VF representors
>>>>>>>    $ tc qdisc add dev enp8s0f0_0 ingress
>>>>>>>    $ tc qdisc add dev enp8s0f0_1 ingress
>>>>>>>    $ ifconfig enp8s0f0_0 up
>>>>>>>    $ ifconfig enp8s0f0_1 up
>>>>>>>
>>>>>>>    #Original side
>>>>>>>    $ tc filter add dev enp8s0f0_0 ingress chain 0 proto ip flower \
>>>>>>>       ct_state -trk ip_proto tcp dst_port 8888 \
>>>>>>>         action pedit ex munge tcp dport set 5001 pipe \
>>>>>>>         action csum ip tcp pipe \
>>>>>>>         action ct pipe \
>>>>>>>         action goto chain 1
>>>>>>>    $ tc filter add dev enp8s0f0_0 ingress chain 1 proto ip flower \
>>>>>>>       ct_state +trk+est \
>>>>>>>         action mirred egress redirect dev enp8s0f0_1
>>>>>>>    $ tc filter add dev enp8s0f0_0 ingress chain 1 proto ip flower \
>>>>>>>       ct_state +trk+new \
>>>>>>>         action ct commit pipe \
>>>>>>>         action mirred egress redirect dev enp8s0f0_1
>>>>>>>    $ tc filter add dev enp8s0f0_0 ingress chain 0 proto arp flower \
>>>>>>>         action mirred egress redirect dev enp8s0f0_1
>>>>>>>
>>>>>>>    #Reply side
>>>>>>>    $ tc filter add dev enp8s0f0_1 ingress chain 0 proto arp flower \
>>>>>>>         action mirred egress redirect dev enp8s0f0_0
>>>>>>>    $ tc filter add dev enp8s0f0_1 ingress chain 0 proto ip flower \
>>>>>>>       ct_state -trk ip_proto tcp \
>>>>>>>         action ct pipe \
>>>>>>>         action pedit ex munge tcp sport set 8888 pipe \
>>>>>>>         action csum ip tcp pipe \
>>>>>>>         action mirred egress redirect dev enp8s0f0_0
>>>>>>>
>>>>>>>    #Run traffic
>>>>>>>    $ ip netns exec ns1 iperf -s -p 5001&
>>>>>>>    $ sleep 2 #wait for iperf to fully open
>>>>>>>    $ ip netns exec ns0 iperf -c 1.1.1.2 -p 8888
>>>>>>>
>>>>>>>    #dump tc filter stats on enp8s0f0_0 chain 0 rule and see hardware packets:
>>>>>>>    $ tc -s filter show dev enp8s0f0_0 ingress chain 0 proto ip | grep "hardware.*pkt"
>>>>>>>           Sent hardware 9310116832 bytes 6149672 pkt
>>>>>>>           Sent hardware 9310116832 bytes 6149672 pkt
>>>>>>>           Sent hardware 9310116832 bytes 6149672 pkt
>>>>>>>
>>>>>>> A new connection executing the first filter in hardware will first rewrite
>>>>>>> the dst port to the new port, and then the ct action is executed,
>>>>>>> because this is a new connection, hardware will need to be send this back
>>>>>>> to software, on chain 0, to execute the first filter again in software.
>>>>>>> The dst port needs to be reverted otherwise it won't re-match the old
>>>>>>> dst port in the first filter. Because of that, currently mlx5 driver will
>>>>>>> reject offloading the above action ct rule.
>>>>>>>
>>>>>>> This series adds supports partial offload of a filter's action list,
>>>>>>> and letting tc software continue processing in the specific action instance
>>>>>>> where hardware left off (in the above case after the "action pedit ex munge tcp
>>>>>>> dport... of the first rule") allowing support for scenarios such as the above.
>>>>>>
>>>>>>
>>>>>> Hi, Paul.  Not sure if this was discussed before, but don't we also need
>>>>>> a new TCA_CLS_FLAGS_IN_HW_PARTIAL flag or something like this?
>>>>>>
>>>>>> Currently the in_hw/not_in_hw flags are reported per filter, i.e. these
>>>>>> flags are not per-action.  This may cause confusion among users, if flows
>>>>>> are reported as in_hw, while they are actually partially or even mostly
>>>>>> processed in SW.
>>>>>>
>>>>>> What do you think?
>>>>>>
>>>>>> Best regards, Ilya Maximets.
>>>>>
>>>>> I think its a good idea, and I'm fine with proposing something like this in a
>>>>> different series, as this isn't a new problem from this series and existed before
>>>>> it, at least with CT rules.
>>>>
>>>> Hmm, I didn't realize the issue already exists.
>>>
>>> Maintainers: please give me up to Friday to review this patchset.
>>>
>>> Disclaimer: I had missed this patchset, and I didn't even read it yet.
>>>
>>> I don't follow. Can someone please rephase the issue please?
>>> AFAICT, it is not that the NIC is offloading half of the action list
>>> and never executing a part of it. Instead, for established connections
>>> the rule will work fully offloaded. While for misses in the CT action,
>>> it will simply trigger a miss, like it already does today.
>>
>> You got it right, and like you said it was like this before so its not
>> strictly related by this series and could be in a different patchset. And I
>> thought that (extra) flag would mean that it can miss, compared to other
>> rules/actions combination that will never miss because they
>> don't need sw support.
> 
> This is different from what I understood from Ilya's comment. Maybe I
> got his comment wrong, but I have the impression that he meant it in
> the sense of having some actions offloaded and some not.
> Which I thinkit is not the goal here.

I don't really know the code around this patch set well enough, so my
thoughts might be a bit irrelevant.  But after reading the cover letter
and commit messages in this patch set I imagined that if we have some
kind of miss on the N-th action in a list in HW, we could go to software
tc, find that action and continue execution from it.  In this case some
actions are executed in HW and some are in SW.

>From the user's perspective, if such tc filter reports an 'in_hw' flag,
that would be a bit misleading, IMO.

If that is not what is happening here, then please ignore my comments,
as I'm not sure what this code is about then. :)

> 
> But anyway, flows can have some packets matching in sw while also
> being in hw. That's expected. For example, in more complex flow sets,
> if a packet hit a flow with ct action and triggered a miss, all
> subsequent flows will handle this packet in sw. Or if we have queued
> packets in rx ring already and ovs just updated the datapath, these
> will match in tc sw instead of going to upcall. The latter will have
> only a few hits, yes, but the former will be increasing over time.
> I'm not sure how a new flag, which is probably more informative than
> an actual state indication, would help here.

These cases are related to just one or a very few packets, so for them
it's generally fine to report 'in_hw', I think.  The vast majority of
traffic will be handled in HW.

My thoughts were about a case where we have a lot of traffic handled
partially in HW and in SW.  Let's say we have N actions and HW doesn't
support action M.  In this case, driver may offload actions [0, M - 1]
inserting some kind of forced "HW miss" at the end, so actions [M, N]
can be executed in TC software.

But now I'm not sure if that is possible with the current implementation.

> 
>>
>>>
>>>>
>>>>>
>>>>> So how about I'll propose it in a different series and we continue with this first?
>>>
>>> So I'm not sure either on what's the idea here.
>>>
>>> Thanks,
>>> Marcelo
>>>
>>>>
>>>> Sounds fine to me.  Thanks!
>>>>
>>>> Best regards, Ilya Maximets.
>>>>
>>>
>>
> 

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