Re: [dpdk-dev] [RFC 0/2] add Tx prepare support for bonding device

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

> -----Original Message-----
> From: Chengchang Tang <tangchengchang@huawei.com>
> Sent: Tuesday, April 20, 2021 1:44 PM
> To: Ananyev, Konstantin <konstantin.ananyev@intel.com>; Yigit, Ferruh <ferruh.yigit@intel.com>; dev@dpdk.org
> Cc: linuxarm@huawei.com; chas3@att.com; humin29@huawei.com
> Subject: Re: [dpdk-dev] [RFC 0/2] add Tx prepare support for bonding device
> 
> Hi
> On 2021/4/20 16:33, Ananyev, Konstantin wrote:
> > Hi everyone,
> >
> >>
> >> On 2021/4/20 9:26, Ferruh Yigit wrote:
> >>> On 4/16/2021 12:04 PM, Chengchang Tang wrote:
> >>>> This patch add Tx prepare for bonding device.
> >>>>
> >>>> Currently, the bonding driver has not implemented the callback of
> >>>> rte_eth_tx_prepare function. Therefore, the TX prepare function of the
> >>>> slave devices will never be invoked. When hardware offloading such as
> >>>> CKSUM and TSO are enabled for some drivers, tx_prepare needs to be used
> >>>> to adjust packets (for example, set correct pseudo packet headers).
> >>>> Otherwise, related offloading fails and even packets are sent
> >>>> incorrectly. Due to this limitation, the bonded device cannot use these
> >>>> HW offloading in the Tx direction.
> >>>>
> >>>> Because packet sending algorithms are numerous and complex in bond PMD,
> >>>> it is hard to design the callback for rte_eth_tx_prepare. In this patch,
> >>>> the tx_prepare callback of bonding PMD is not implemented. Instead,
> >>>> rte_eth_tx_prepare has been called in tx_burst callback. And a global
> >>>> variable is introduced to control whether the bonded device need call
> >>>> the rte_eth_tx_prepare. If upper-layer users need to use some TX
> >>>> offloading that depend on tx_prepare , they should enable the preparation
> >>>> function. In this way, the bonded device will call the rte_eth_tx_prepare
> >>>> for the fast path packets in the tx_burst callback.
> >
> > I admit that I didn't look at the implementation yet, but it sounds like
> > overcomplication to me. Can't we just have a new TX function for bonding PMD
> > when TX offloads are enabled? And inside that function we will do:
> > tx_prepare(); tx_burst(); for selected device.
> 
> The solution you mentioned is workable and may perform better. However, the current
> solution is also simple and has a limited impact on performance. It is actually:
> if (tx_prepare_enable)
> 	tx_prepare();
> tx_burst();
> 
> Overall, it adds almost only one judgment to the case where the related Tx offloads
> is not turned on.
> 
> > We can select this function at setup stage analysing requested by user TX offloads.
> >
> 
> In PMDs, it is a common practice to select different Tx/Rx function during the setup
> phase. But for a 'vdev' device like Bonding, we may need to think more about it.
> The reasons are explained below.
> >
> >>>>
> >>>
> >>> What do you think to add a devarg to bonding PMD to control the tx_prepare?
> >>> It won't be as dynamic as API, since it can be possible to change the behavior after application is started with API, but do we really need
> >> this?
> >>
> >> If an API is not added, unnecessary constraints may be introduced. If the
> >> bonding device is created through the rte_eth_bond_create interface instead
> >> devarg "vdev", this function cannot be used because devargs does not take effect
> >> in this case. But from an ease-of-use perspective, adding a devarg is a good
> >> idea. I will add related implementations in the later official patches.
> >
> > I am also against introducing new devarg to control tx_prepare() invocation.
> > I think at dev_config/queue_setup phase PMD will have enough information to decide.
> >
> Currently, the community does not specify which Tx offloads need to invoke tx_prepare.

I think inside bond PMD we can safely assume that any TX offload does need tx_prepare().
If that's not the case then slave dev tx_prepare pointer will be NULL and rte_eth_tx_prepare()
will be just a NOOP. 

> For Vdev devices such as bond, all NIC devices need to be considered. Generally,
> tx_prepare is used in CKSUM and TSO. It is possible that for some NIC devices, even
> CKSUM and TSO do not need to invoke tx_prepare, or for some NIC devices, there are
> other Tx offloads that need to call tx_prepare. From this perspective, leaving the
> choice to the user seems to be a better choice.

Wonder how user will know when to enable/disable it?
As you said it depends on the underlying HW/PMD and can change from system to system?
I think it is PMD that needs to take this decision, and I think the safest bet might be to enable
it when any TX offloads was enabled by user.

> >>
> >> If I understand correctly, the current community does not want to introduce
> >> more private APIs for PMDs. However, the absence of an API on this issue would
> >> introduce some unnecessary constraints, and from that point of view, I think
> >> adding an API seems necessary.
> >>>
> >>>> Chengchang Tang (2):
> >>>>    net/bonding: add Tx prepare for bonding
> >>>>    app/testpmd: add cmd for bonding Tx prepare
> >>>>
> >>>>   app/test-pmd/cmdline.c                      | 66 +++++++++++++++++++++++++++++
> >>>>   doc/guides/testpmd_app_ug/testpmd_funcs.rst |  9 ++++
> >>>>   drivers/net/bonding/eth_bond_private.h      |  1 +
> >>>>   drivers/net/bonding/rte_eth_bond.h          | 29 +++++++++++++
> >>>>   drivers/net/bonding/rte_eth_bond_api.c      | 28 ++++++++++++
> >>>>   drivers/net/bonding/rte_eth_bond_pmd.c      | 33 +++++++++++++--
> >>>>   drivers/net/bonding/version.map             |  5 +++
> >>>>   7 files changed, 167 insertions(+), 4 deletions(-)
> >>>>
> >>>> --
> >>>> 2.7.4
> >>>>
> >>>
> >>>
> >>> .
> >>>
> >