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

["Morten Brørup" <mb@smartsharesystems.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
> > >
> > > >
> > > > > 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.
> >
> > I do not consider these to be core libraries in DPDK. If these
> libraries are used in an application, that application must be compiled
> with
> > RTE_ETHDEV_RXTX_CALLBACKS.
> >
> > > With the approach above their functionality will be broken -
> > > setup functions will return success, but actual callbacks will not
> be
> > > invoked.
> >
> > I just took a look at bpf and latencystat. Bpf correctly checks for
> the return code, and returns an error if ethdev has been compiled
> without
> > RTE_ETHDEV_RXTX_CALLBACKS. Latencystat checks for the return code,
> but only logs the error and continues as if everything is good
> > anyway. That is a bug in the latencystat library.
> 
> If RTE_ETHDEV_RXTX_CALLBACKS Is enabled or disabled for both DPDK and
> user app - everything will work as expected.
> But, as I understand, you consider approach when
> RTE_ETHDEV_RXTX_CALLBACKS Is enabled in the DPDK, but disabled in the
> app.
> Such approach will cause a problems with some  libraries - as I
> outlined above.
> 
> >
> > > 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
> >
> > In principle, I completely agree with your reasoning from a high
> level perspective.
> >
> > However, accepting it would probably lead to the
> RTE_ETHDEV_RXTX_CALLBACKS compile time configuration option being
> completely
> > removed,
> 
> For now, we are not talking about removing or even deprecating
> RTE_ETHDEV_RXTX_CALLBACKS.
> What I am talking about - user has to use it (and other rte_config.h
> options) properly.
> He can't use different configs for DPDK libs and app and expect things
> 'just work'.
> This is not supported right now, I think it will never be.
> If it works right now, this is just implementation specifics, which
> user should not rely on.

I agree.

> 
> > and ethdev callbacks being always supported. And I don't think such a
> performance degradation of a core DPDK library should be
> > accepted.
> 
> As I said above, RTE_ETHDEV_RXTX_CALLBACKS Is still there.
> If it is really critical for your app to disable it - you can still do
> it, you just need to do it in a proper way.

I hope so. This is exactly what I am pleading for: Keep the ability to disable RTE_ETHDEV_RXTX_CALLBACKS at compile time, so there is no performance impact for applications not using it.

I also agree with the limitation that both library and application should be compiled with the same configuration.

> 
> > <rant on>
> > I was opposed to the "callback hooks" concept from the beginning, and
> still am. The path that packets take through various functions and
> > pipeline stages should be determined and implemented by the
> application, not by the DPDK libraries.
> >
> > If we want to provide a standardized advanced IP pipeline for DPDK,
> we could offer it as a middle layer library using the underlying DPDK
> > libraries to implement various callbacks, IP fragmentation
> reassembly, etc.. Don't tweak the core libraries (costing memory and/or
> > performance) to support an increasing amount of supplemental
> libraries, which may not be used by all applications.
> >
> > We don't want DPDK to become like the Linux IP stack, with callback
> hooks and runtime installable protocol handling everywhere. All this
> > cruft with their small performance degradations adds up!
> > <rant off>
> >
> > >
> > > >
> > > > 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.