From: Jack Min <jackmin@nvidia.com>
To: Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>,
Stephen Hemminger <stephen@networkplumber.org>
Cc: "dev@dpdk.org" <dev@dpdk.org>, Matan Azrad <matan@nvidia.com>,
"viacheslavo@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
Date: Fri, 18 Aug 2023 13:57:21 +0800 [thread overview]
Message-ID: <1fbe4f48-e652-4887-8949-ad9cd15e2c26@nvidia.com> (raw)
In-Reply-To: <DBAPR08MB58141E46D6EFB767929E4453981BA@DBAPR08MB5814.eurprd08.prod.outlook.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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next prev parent reply other threads:[~2023-08-18 5:57 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-08-17 5:06 Honnappa Nagarahalli
2023-08-17 14:06 ` Stephen Hemminger
2023-08-18 2:32 ` Jack Min
2023-08-18 4:30 ` Honnappa Nagarahalli
2023-08-18 5:57 ` Jack Min [this message]
2023-08-18 13:59 ` Honnappa Nagarahalli
2023-08-19 1:34 ` Jack Min
2023-08-21 6:06 ` Honnappa Nagarahalli
2023-08-21 6:56 ` Jack Min
2023-08-22 4:18 ` Honnappa Nagarahalli
2023-08-18 9:05 ` Konstantin Ananyev
2023-08-18 9:38 ` Jack Min
2023-08-19 11:57 ` Konstantin Ananyev
2023-08-20 1:41 ` Jack Min
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