MLX5 PMD access ring library private data

[Honnappa Nagarahalli]

It would be good if you could fix the email on your side to text format. Comments inline.

From: Jack Min <jackmin at nvidia.com>
Sent: Friday, August 18, 2023 8:35 PM
To: Honnappa Nagarahalli <Honnappa.Nagarahalli at arm.com>; Stephen Hemminger <stephen at networkplumber.org>
Cc: dev at dpdk.org; Matan Azrad <matan at nvidia.com>; viacheslavo at nvidia.com; Tyler Retzlaff <roretzla at linux.microsoft.com>; Wathsala Wathawana Vithanage <wathsala.vithanage at arm.com>; nd <nd at arm.com>
Subject: Re: MLX5 PMD access ring library private data

On 2023/8/18 21:59, Honnappa Nagarahalli wrote:
From: Jack Min <jackmin at nvidia.com><mailto:jackmin at nvidia.com>
Sent: Friday, August 18, 2023 12:57 AM
To: Honnappa Nagarahalli <Honnappa.Nagarahalli at arm.com><mailto:Honnappa.Nagarahalli at arm.com>; Stephen Hemminger <stephen at networkplumber.org><mailto:stephen at networkplumber.org>
Cc: dev at dpdk.org<mailto:dev at dpdk.org>; Matan Azrad <matan at nvidia.com><mailto:matan at nvidia.com>; viacheslavo at nvidia.com<mailto:viacheslavo at nvidia.com>; Tyler Retzlaff <roretzla at linux.microsoft.com><mailto:roretzla at linux.microsoft.com>; Wathsala Wathawana Vithanage <wathsala.vithanage at arm.com><mailto:wathsala.vithanage at arm.com>; nd <nd at arm.com><mailto:nd at arm.com>
Subject: Re: MLX5 PMD access ring library private data

On 2023/8/18 12:30, Honnappa Nagarahalli wrote:





-----Original Message-----

From: Jack Min <jackmin at nvidia.com><mailto:jackmin at nvidia.com>

Sent: Thursday, August 17, 2023 9:32 PM

To: Stephen Hemminger <stephen at networkplumber.org><mailto:stephen at networkplumber.org>; Honnappa

Nagarahalli <Honnappa.Nagarahalli at arm.com><mailto:Honnappa.Nagarahalli at arm.com>

Cc: dev at dpdk.org<mailto:dev at dpdk.org>; Matan Azrad <matan at nvidia.com><mailto:matan at nvidia.com>;

viacheslavo at nvidia.com<mailto:viacheslavo at nvidia.com>; Tyler Retzlaff <roretzla at linux.microsoft.com><mailto:roretzla at linux.microsoft.com>;

Wathsala Wathawana Vithanage <wathsala.vithanage at arm.com><mailto:wathsala.vithanage at arm.com>; nd

<nd at arm.com><mailto:nd at 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 at arm.com><mailto:Honnappa.Nagarahalli at 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).

[Honnappa] May be I was not clear in my recommendation. What I am saying is, you could call ‘rte_ring_free_count’ to check if you have enough space on the cache ring. If there is not enough space you can enqueue the new objects on the global ring. Pseudo code below:

If (rte_ring_free_count(cache_ring) > n) {

             <enqueue n objects on cache ring>

} else {

             <enqueue n objects on global ring>

}

Hey,

Then next n objects will still enqueue into global ring, not into cache , right? ( we enqueue nnnn objects continually)

Our requirement is like this:

if (rte_ring_free_count(cache_ring) > 0) {

         <enqueue this object on cache ring>

} else { /* cache is full */

      <enqueue this object into global ring>

     <move the latest n objects into global ring too>

[Honnappa] Understood. IMO, this is a unique requirement. Ring library does not support this and is not designed for this. As per the guidelines and past agreements, accessing structure members in ring structures is not allowed.

I think a simple implementation like [1] would suffice your needs.

[1] https://patches.dpdk.org/project/dpdk/patch/20230821060420.3509667-1-honnappa.nagarahalli@arm.com/

}

It's not about if this enqueue on cache can success or not.

It's about we need "free" more room in advance so next n objects can enqueue into cache.

-Jack







-Jack




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