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

["Ananyev, Konstantin" <konstantin.ananyev@intel.com>]

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.