Re: MLX5 PMD access ring library private data

[Jack Min <jackmin@nvidia.com>]

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On 2023/8/18 12:30, Honnappa Nagarahalli wrote:
>
>> -----Original Message-----
>> From: Jack Min<jackmin@nvidia.com>
>> Sent: Thursday, August 17, 2023 9:32 PM
>> To: Stephen Hemminger<stephen@networkplumber.org>; Honnappa
>> Nagarahalli<Honnappa.Nagarahalli@arm.com>
>> Cc:dev@dpdk.org; Matan Azrad<matan@nvidia.com>;
>> viacheslavo@nvidia.com; Tyler Retzlaff<roretzla@linux.microsoft.com>;
>> Wathsala Wathawana Vithanage<wathsala.vithanage@arm.com>; nd
>> <nd@arm.com>
>> Subject: Re: MLX5 PMD access ring library private data
>>
>> On 2023/8/17 22:06, Stephen Hemminger wrote:
>>> On Thu, 17 Aug 2023 05:06:20 +0000
>>> Honnappa Nagarahalli<Honnappa.Nagarahalli@arm.com>  wrote:
>>>
>>>> Hi Matan, Viacheslav,
>>>> 	Tyler pointed out that the function
>> __mlx5_hws_cnt_pool_enqueue_revert is accessing the ring private structure
>> members (prod.head and prod.tail) directly. Even though ' struct rte_ring' is a
>> public structure (mainly because the library provides inline functions), the
>> structure members are considered private to the ring library. So, this needs to
>> be corrected.
>>>> It looks like the function __mlx5_hws_cnt_pool_enqueue_revert is trying
>> to revert things that were enqueued. It is not clear to me why this
>> functionality is required. Can you provide the use case for this? We can
>> discuss possible solutions.
>>> How can reverting be thread safe? Consumer could have already looked at
>> them?
>>
>> Hey,
>>
>> In our case, this ring is SC/SP, only accessed by one thread
>> (enqueue/dequeue/revert).
> You could implement a more simpler and more efficient (For ex: such an implementation would not need any atomic operations, would require less number of cache lines) ring for this.
> Is this function being used in the dataplane?

Yes,  we can have our own version of ring (no atomic operations) but 
basic operation are still as same as rte_ring.

Since rte ring has been well-designed and tested sufficiently, so there 
is no strong reason to re-write a new simple version of it until today :)


>
>> The scenario we have "revert" is:
>>
>>    We use ring to manager our HW objects (counter in this case) and for each
>> core (thread) has "cache" (a SC/SP ring) for sake of performance.
>>
>> 1. Get objects from "cache" firstly, if cache is empty, we fetch a bulk of free
>> objects from global ring into cache.
>>
>> 2. Put (free) objects also into "cache" firstly, if cache is full, we flush a bulk of
>> objects into global ring in order to make some rooms in cache.
>>
>> However, this HW object cannot be immediately reused after free. It needs
>> time to be reset and then can be used again.
>>
>> So when we flush cache, we want to keep the first enqueued objects still stay
>> there because they have more chance already be reset than the latest
>> enqueued objects.
>>
>> Only flush recently enqueued objects back into global ring, act as "LIFO"
>> behavior.
>>
>> This is why we require "revert" enqueued objects.
> You could use 'rte_ring_free_count' API before you enqueue to check for available space.

Only when cache is full (rte_ring_free_count() is zero), we revert X 
objects.

If there is still  one free slot we will not trigger revert (flush).


>
>> -Jack
>>

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