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

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

> 
> > From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Ananyev,
> > Konstantin
> >
> > >
> > > > From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Ananyev,
> > > > Konstantin
> > > >
> > > > > > 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;
> > > >  }
> > >
> > > That would make the compiler inline _rte_eth_rx_epilog() into the
> > driver when compiling the DPDK library. But
> > > RTE_ETHDEV_RXTX_CALLBACKS is a definition for the application
> > developer to use when compiling the DPDK application.
> >
> > I believe it is for both - user app and DPDK drivers.
> > AFAIK, they both have to use the same rte_config.h, otherwise things
> > will be broken.
> > If let say RTE_ETHDEV_RXTX_CALLBACKS is not enabled in ethdev, then
> > user wouldn't be able to add a callback at first place.
> 
> In the case of RTE_ETHDEV_RXTX_CALLBACKS, it is independent:

Not really.
There are few libraries within DPDK that do rely on rx/tx callbacks:
bpf, latencystat, pdump, power.
With the approach above their functionality will be broken -
setup functions will return success, but actual callbacks will not be invoked. 
From other side, some libraries do invoke rx/tx burst on their own: ip-pipeline, graph.
For them callback invocation will continue to work, even when
RTE_ETHDEV_RXTX_CALLBACKS is disabled in the app.
In general, building DPDK libs and user app with different rte_config.h is really a bad idea.
It might work in some cases, but I believe it is not supported and user should not rely on it.
If user needs to disable RTE_ETHDEV_RXTX_CALLBACKS in his app, then the proper way would be:
- update rte_config.h
- rebuild both DPDK and the app with new config

> 
> If it is not compiled with the DPDK library, attempts to install callbacks from the application will fail with ENOTSUP.
> 
> If it is not compiled with the DPDK application, no time will be spent trying to determine if any there are any callbacks to call.
> 
> > BTW,  such change will allow us to make RTE_ETHDEV_RXTX_CALLBACKS
> > internal for ethdev/PMD layer, which is a good thing from my
> > perspective.
> 
> If it can be done without degrading performance for applications not using callbacks.