Re: MLX5 PMD access ring library private data

[Konstantin Ananyev <konstantin.v.ananyev@yandex.ru>]

18/08/2023 10:38, Jack Min пишет:
> On 2023/8/18 17:05, Konstantin Ananyev wrote:
>>> 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).
>>>
>>> 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.
>>>
>> Wouldn't then simple stack fit you better?
>> Something like lib/stack/rte_stack_std.h, but even without spinlock around?
> 
> No, stack is always a "LIFO" struct, right?

Yep.

> 
> Here first we need this cache works as "FIFO" in most cases (get/put) 
> because the first enqueued objects have more chance that are already 
> reset so can reuse them.
> 
> We only require "LIFO" behavior when "flush" cache in order to make some 
> room, so next free will be quick because it happens in our local cache, 
> needn't access global ring.
> 
> In short, we require a struct supports "FIFO" and "LIFO".

Ok, thanks fro explanation.
So you need a ring, but with an ability to revert prod N elements back, 
right?