Re: [dpdk-dev] [PATCH] parray: introduce internal API for dynamic arrays

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

On 6/21/2021 12:06 PM, Ananyev, Konstantin wrote:
> 
> Hi everyone,
> 
>>>>>> One more thought here - if we are talking about rte_ethdev[] in particular, I think  we can:
>>>>>> 1. move public function pointers (rx_pkt_burst(), etc.) from rte_ethdev into a separate flat array.
>>>>>> We can keep it public to still use inline functions for 'fast' calls rte_eth_rx_burst(), etc. to avoid
>>>>>> any regressions.
>>>>>> That could still be flat array with max_size specified at application startup.
>>>>>> 2. Hide rest of rte_ethdev struct in .c.
>>>>>> That will allow us to change the struct itself and the whole rte_ethdev[] table in a way we like
>>>>>> (flat array, vector, hash, linked list) without ABI/API breakages.
>>>>>>
>>>>>> Yes, it would require all PMDs to change prototype for pkt_rx_burst() function
>>>>>> (to accept port_id, queue_id instead of queue pointer), but the change is mechanical one.
>>>>>> Probably some macro can be provided to simplify it.
>>>>>>
>>>>>
>>>>> We are already planning some tasks for ABI stability for v21.11, I think
>>>>> splitting 'struct rte_eth_dev' can be part of that task, it enables hiding more
>>>>> internal data.
>>>>
>>>> Ok, sounds good.
>>>>
>>>>>
>>>>>> The only significant complication I can foresee with implementing that approach -
>>>>>> we'll need a an array of 'fast' function pointers per queue, not per device as we have now
>>>>>> (to avoid extra indirection for callback implementation).
>>>>>> Though as a bonus we'll have ability to use different RX/TX funcions per queue.
>>>>>>
>>>>>
>>>>> What do you think split Rx/Tx callback into its own struct too?
>>>>>
>>>>> Overall 'rte_eth_dev' can be split into three as:
>>>>> 1. rte_eth_dev
>>>>> 2. rte_eth_dev_burst
>>>>> 3. rte_eth_dev_cb
>>>>>
>>>>> And we can hide 1 from applications even with the inline functions.
>>>>
>>>> As discussed off-line, I think:
>>>> it is possible.
>>>> My absolute preference would be to have just 1/2 (with CB hidden).
>>>
>>> How can we hide the callbacks since they are used by inline burst functions.
>>
>> I probably I owe a better explanation to what I meant in first mail.
>> Otherwise it sounds confusing.
>> I'll try to write a more detailed one in next few days.
> 
> Actually I gave it another thought over weekend, and might be we can
> hide rte_eth_dev_cb even in a simpler way. I'd use eth_rx_burst() as
> an example, but the same principle applies to other 'fast' functions.
> 
>  1. Needed changes for PMDs rx_pkt_burst():
>     a) change function prototype to accept 'uint16_t port_id' and 'uint16_t queue_id',
>          instead of current 'void *'.
>     b) Each PMD rx_pkt_burst() will have to call rte_eth_rx_epilog() function at return.
>          This  inline function will do all CB calls for that queue.
> 
> To be more specific, let say we have some PMD: xyz with RX function:
> 
> uint16_t
> xyz_recv_pkts(void *rx_queue, struct rte_mbuf **rx_pkts, uint16_t nb_pkts)
> {
>      struct xyz_rx_queue *rxq = rx_queue;
>      uint16_t nb_rx = 0;
> 
>      /* do actual stuff here */
>     ....
>     return nb_rx;
> }
> 
> It will be transformed to:
> 
> uint16_t
> xyz_recv_pkts(uint16_t port_id, uint16_t queue_id, struct rte_mbuf **rx_pkts, uint16_t nb_pkts)
> {
>          struct xyz_rx_queue *rxq;
>          uint16_t nb_rx;
> 
>          rxq = _rte_eth_rx_prolog(port_id, queue_id);
>          if (rxq == NULL)
>              return 0;
>          nb_rx = _xyz_real_recv_pkts(rxq, rx_pkts, nb_pkts);
>          return _rte_eth_rx_epilog(port_id, queue_id, rx_pkts, nb_pkts);
> }
> 
> And somewhere in ethdev_private.h:
> 
> static inline void *
> _rte_eth_rx_prolog(uint16_t port_id, uint16_t queue_id);
> {
>    struct rte_eth_dev *dev = &rte_eth_devices[port_id];
> 
> #ifdef RTE_ETHDEV_DEBUG_RX
>         RTE_ETH_VALID_PORTID_OR_ERR_RET(port_id, NULL);
>         RTE_FUNC_PTR_OR_ERR_RET(*dev->rx_pkt_burst, NULL);
> 
>         if (queue_id >= dev->data->nb_rx_queues) {
>                 RTE_ETHDEV_LOG(ERR, "Invalid RX queue_id=%u\n", queue_id);
>                 return NULL;
>         }
> #endif
>   return dev->data->rx_queues[queue_id];
> }
> 
> static inline uint16_t
> _rte_eth_rx_epilog(uint16_t port_id, uint16_t queue_id, struct rte_mbuf **rx_pkts, const uint16_t nb_pkts);
> {
>     struct rte_eth_dev *dev = &rte_eth_devices[port_id];
> 
> #ifdef RTE_ETHDEV_RXTX_CALLBACKS
>         struct rte_eth_rxtx_callback *cb;
> 
>         /* __ATOMIC_RELEASE memory order was used when the
>          * call back was inserted into the list.
>          * Since there is a clear dependency between loading
>          * cb and cb->fn/cb->next, __ATOMIC_ACQUIRE memory order is
>          * not required.
>          */
>         cb = __atomic_load_n(&dev->post_rx_burst_cbs[queue_id],
>                                 __ATOMIC_RELAXED);
> 
>         if (unlikely(cb != NULL)) {
>                 do {
>                         nb_rx = cb->fn.rx(port_id, queue_id, rx_pkts, nb_rx,
>                                                 nb_pkts, cb->param);
>                         cb = cb->next;
>                 } while (cb != NULL);
>         }
> #endif
> 
>         rte_ethdev_trace_rx_burst(port_id, queue_id, (void **)rx_pkts, nb_rx);
>         return nb_rx;
>  }
> 
> Now, as you said above, in rte_ethdev.h we will keep only a flat array
> with pointers to 'fast' functions:
> struct {
>      eth_rx_burst_t             rx_pkt_burst
>       eth_tx_burst_t             tx_pkt_burst;
>       eth_tx_prep_t              tx_pkt_prepare;
>      .....
> } rte_eth_dev_burst[];
> 
> And rte_eth_rx_burst() will look like:
> 
> static inline uint16_t
> rte_eth_rx_burst(uint16_t port_id, uint16_t queue_id,
>                  struct rte_mbuf **rx_pkts, const uint16_t nb_pkts)
> {
>     if (port_id >= RTE_MAX_ETHPORTS)
>         return 0;
>    return rte_eth_dev_burst[port_id](port_id, queue_id, rx_pkts, nb_pkts);
> }
> 
> Yes, it will require changes in *all* PMDs, but as I said before the changes will be a mechanic ones.
> 

I did not like the idea to push to calling Rx/TX callbacks responsibility to the
drivers, I think it should be in the ethdev layer.

What about making 'rte_eth_rx_epilog' an API and call from 'rte_eth_rx_burst()',
which will add another function call for Rx/Tx callback but shouldn't affect the
Rx/Tx burst.