Re: [dpdk-dev] [PATCH RFC 0/3] DPDK ethdev callback support

[Stephen Hemminger <stephen@networkplumber.org>]

On Mon, 22 Dec 2014 16:47:21 +0000
Bruce Richardson <bruce.richardson@intel.com> wrote:

> This RFC is for a small addition to the ethdev library, to add in support for
> callbacks at the RX and TX stages. This allows packet processing to be done on
> packets before they get returned to applications using rte_eth_rx_burst call.
> 
> Use case: the first use case for this is to enable a consistent set of 
> packets mbufs to be received by applications irrespective of the NIC used
> to receive those. For example, for a port type that does not support RSS,
> a callback on RX can be configured to calculate a hash in software. 
> Similarly, this mechanism can be used to add other information to mbufs
> as they are received, such as timestamps or sequence numbers, without cluttering
> up the main packet processing path with checks for whether packets have these
> fields filled in or not.
> A second use case is ease of intrumenting existing code. The example application 
> shows how combining a timestamp insertion callback on RX can be paired with a 
> latency calculation callback on TX to easily instrument any application for
> packet latency.
> A third use case is to potentially extend existing NIC capabilities beyond
> what is currently supported. For example, where flow director capabilities
> can match up to a certain limit of flows - in the thousands, in the case of
> NICs using the ixgbe driver - a callback can extend this to potentially
> millions of flows by using a software hash table lookup inline for packets
> that missing the hardware lookup filters. It would all appear transparent 
> to the packet handling code in the main application.
> 
> Future extensions: in future the ethdev library can be extended to provide
> a standard set of callbacks for use by drivers. 
> 
> For now this patch set is RFC and still needs additional work for creating
> a remove function for callbacks and to add in additional testing code.
> Since this adds in new code into the critical data path, I have run some
> performance tests using testpmd with the ixgbe vector drivers (i.e. the
> fastest, fast-path we have :-) ). Performance drops due to this patch
> seems minimal to non-existant, rough tests on my system indicate a drop
> of perhaps 1%.
> 
> All feedback welcome.
> 
> Bruce Richardson (3):
>   ethdev: rename callbacks field to intr_cbs
>   ethdev: Add in data rxtx callback support
>   examples: example showing use of callbacks.
> 
>  app/test/virtual_pmd.c                 |   2 +-
>  examples/rxtx_callbacks/Makefile       |  57 +++++++++
>  examples/rxtx_callbacks/basicfwd.c     | 222 +++++++++++++++++++++++++++++++++
>  examples/rxtx_callbacks/basicfwd.h     |  46 +++++++
>  lib/librte_ether/rte_ethdev.c          | 103 +++++++++++++--
>  lib/librte_ether/rte_ethdev.h          | 125 ++++++++++++++++++-
>  lib/librte_pmd_bond/rte_eth_bond_api.c |   2 +-
>  7 files changed, 543 insertions(+), 14 deletions(-)
>  create mode 100644 examples/rxtx_callbacks/Makefile
>  create mode 100644 examples/rxtx_callbacks/basicfwd.c
>  create mode 100644 examples/rxtx_callbacks/basicfwd.h
> 

What about SMP safety? The callback list is not thread safe.
Do you plan to start integrating with an RCU framework like userspace RCU?