Re: [RFC] ethdev: fast path async flow API

[Ferruh Yigit <ferruh.yigit@amd.com>]

On 1/30/2024 12:06 PM, Dariusz Sosnowski wrote:
> Hi Ferruh,
> 
>> -----Original Message-----
>> From: Ferruh Yigit <ferruh.yigit@amd.com>
>> Sent: Monday, January 29, 2024 18:36
>> To: Dariusz Sosnowski <dsosnowski@nvidia.com>; Stephen Hemminger
>> <stephen@networkplumber.org>
>> Cc: NBU-Contact-Thomas Monjalon (EXTERNAL) <thomas@monjalon.net>;
>> Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>; Ori Kam
>> <orika@nvidia.com>; dev@dpdk.org
>> Subject: Re: [RFC] ethdev: fast path async flow API
>>
>> On 1/29/2024 1:38 PM, Dariusz Sosnowski wrote:
>>> Hi all,
>>>
>>>> -----Original Message-----
>>>> From: Dariusz Sosnowski <dsosnowski@nvidia.com>
>>>> Sent: Tuesday, January 23, 2024 12:37
>>>> To: Stephen Hemminger <stephen@networkplumber.org>
>>>> Cc: NBU-Contact-Thomas Monjalon (EXTERNAL) <thomas@monjalon.net>;
>>>> Ferruh Yigit <ferruh.yigit@amd.com>; Andrew Rybchenko
>>>> <andrew.rybchenko@oktetlabs.ru>; Ori Kam <orika@nvidia.com>;
>>>> dev@dpdk.org
>>>> Subject: RE: [RFC] ethdev: fast path async flow API
>>>>
>>>> Hi Stephen,
>>>>
>>>> Sorry for such a late response.
>>>>
>>>> From: Stephen Hemminger <stephen@networkplumber.org>
>>>> Sent: Thursday, January 4, 2024 02:08
>>>>> On Wed, 3 Jan 2024 19:14:49 +0000
>>>>> Dariusz Sosnowski <dsosnowski@nvidia.com> wrote:
>>>>>> In summary, in my opinion extending the async flow API with bulking
>>>>> capabilities or exposing the queue directly to the application is not
>> desirable.
>>>>>> This proposal aims to reduce the I-cache overhead in async flow API
>>>>>> by
>>>>> reusing the existing design pattern in DPDK - fast path functions
>>>>> are inlined to the application code and they call cached PMD callbacks.
>>>>>
>>>>> Inline needs to more discouraged in DPDK, because it only works if
>>>>> application ends up building with DPDK from source.
>>>>> It doesn't work for the Linux distro packaging model and symbol
>>>>> versioning, etc.
>>>>
>>>> I understand the problem. In that case, I have a proposal.
>>>> I had a chat with Thomas regarding this RFC, and he noticed that
>>>> there are 2 separate changes proposed here:
>>>>
>>>> 1. Per-port callbacks for async flow API.
>>>>     - Moves specified callbacks to rte_flow_fp_ops struct and allow
>>>> PMDs to register them dynamically.
>>>>     - Removes indirection at API level - no need to call rte_flow_ops_get().
>>>>     - Removes checking if callbacks are defined - either the default
>>>> DPDK callback is used or the one provided by PMD.
>>>> 2. Make async flow API functions inlineable.
>>>>
>>>> Change (1) won't break ABI (existing callbacks in rte_flow_ops can be
>>>> marked as deprecated for now and phased out later) and in my opinion
>>>> is less controversial compared to change (2).
>>>>
>>>> I'll rerun the benchmarks for both changes separately to compare
>>>> their benefits in isolation.
>>>> This would allow us to decide if change (2) is worth the drawbacks it
>>>> introduces.
>>>>
>>>> What do you think?
>>>
>>> I split the RFC into 2 parts:
>>>
>>> 1. Introduce per-port callbacks:
>>>     - Introduce rte_flow_fp_ops struct - holds callbacks for fast path calls, for
>> each port. PMD registers callbacks through rte_flow_fp_ops_register().
>>>     - Relevant rte_flow_async_* functions just pass arguments to fast path
>> callbacks. Validation checks are done only if RTE_FLOW_DEBUG macro is
>> defined.
>>>     - Biggest difference is the removal of callback access through
>> rte_flow_get_ops().
>>> 2. Inline async flow API functions.
>>>     - Relevant rte_flow_async_* functions definitions are moved to rte_flow.h
>> and made inlineable.
>>>
>>> Here are the results of the benchmark:
>>>
>>> |                       | Avg Insertion | Diff over baseline | Avg
>>> | Deletion | Diff over baseline |
>>> |-----------------------|---------------|--------------------|--------------|-------------
>> -------|
>>> | baseline (v24.03-rc0) |     5855.4    |                    |    9026.8    |                    |
>>> | applied (1)           |     6384.2    |    +528.8 (+9%)    |    10054.2   |  +1027.4
>> (+11.4%)  |
>>> | applied (2)           |     6434.6    |   +579.2 (+9.9%)   |    10011.4   |   +984.6
>> (+10.9%)  |
>>>
>>> Results are in KFlows/sec.
>>> The benchmark is continuously inserting and deleting 2M flow rules.
>>> These rules match on IPv4 destination address and with a single action
>> DROP.
>>> Flow rules are inserted and deleted using a single flow queue.
>>>
>>> Change (2) improves insertion rate performance by ~1% compared to (1),
>> but decreases deletion rate by ~0.5%.
>>> Based on these results, I think we can say that making rte_flow_async_*()
>> calls inlineable may not be worth it compared to the issues it causes.
>>>
>>> What do you all think about the results?
>>>
>>
>> Hi Dariusz,
>>
>> As discussed before, converting APIs to static inline functions or exposing
>> 'rte_eth_dev' has cons but with only applying first part above seems get rid of
>> them with reasonable performance gain, so I think we can continue with this
>> approach and continue to reviews.
>>
>> Most of the 'rte_flow_async_*' are already missing the function validity check,
>> so having a default (dummy?) rte_flow_fp_ops for them seems even an
>> improvement there.
>>
>>
>> Only previously 'struct rte_flow_ops' was coming from drivers, ethdev layer
>> doesn't need to maintain anything.
>> But with 'rte_flow_fp_ops' struct, ethdev needs to store another array with
>> fixed ('RTE_MAX_ETHPORTS') size which will be another blocker in the future
>> to convert this fixed arrays to dynamically allocated arrays.
>>
>> For this, does it still help we add an a new field to "struct rte_eth_dev", like
>> "struct rte_flow_ops *flow_ops", similar to 'dev_ops'?
>> The indirection still will be there, but eliminate 'rte_flow_get_ops()'
>> call and checks comes with it.
>> If makes sense, is there a chance to experiment this and get some performance
>> numbers with it?
> Which option do you have in mind specifically?
> 
> 1. Keeping only fast path callbacks in "dev->flow_ops", so "struct rte_eth_dev" will hold only a pointer to "struct rte_flow_fp_ops" as defined in RFC.
>     - Only async flow API will use "dev->flow_ops->callback".
>     - Other APIs will go through "rte_flow_ops_get()"
> 2. Keeping all flow callbacks in "dev->flow_ops", so "struct rte_flow_ops".
>     - As a result, I think that "rte_flow_get_ops()" could be removed altogether assuming that all functions have default implementation,
>       checking for port validity (ENODEV if port not valid) and returning ENOSYS.
> 
> Regardless of the version, I can experiment with additional indirection.
> 
> 

Both can work, has some cons,

I think "rte_flow_ops_get()" is more clear approach and rte_flow APIs
already using it, so no change is required, but using separate struct
for fast path will create two different structs drivers needs to fill,
and both uses slightly different way to populate which is not nice.

For this RFC I think we can go with option 1, and consider updating rest
if there is more motivation for it.