Re: [dpdk-dev] [PATCH v7 1/2] mbuf: introduce accurate packet Tx scheduling

[Slava Ovsiienko <viacheslavo@mellanox.com>]

> -----Original Message-----
> From: dev <dev-bounces@dpdk.org> On Behalf Of Viacheslav Ovsiienko
> Sent: Friday, July 10, 2020 15:40
> To: dev@dpdk.org
> Cc: Matan Azrad <matan@mellanox.com>; Raslan Darawsheh
> <rasland@mellanox.com>; olivier.matz@6wind.com;
> arybchenko@solarflare.com; Thomas Monjalon <thomas@monjalon.net>;
> ferruh.yigit@intel.com
> Subject: [dpdk-dev] [PATCH v7 1/2] mbuf: introduce accurate packet Tx
> scheduling
> 
> There is the requirement on some networks for precise traffic timing
> management. The ability to send (and, generally speaking, receive) the
> packets at the very precisely specified moment of time provides the
> opportunity to support the connections with Time Division Multiplexing using
> the contemporary general purpose NIC without involving an auxiliary
> hardware. For example, the supporting of O-RAN Fronthaul interface is one
> of the promising features for potentially usage of the precise time
> management for the egress packets.
> 
> The main objective of this patchset is to specify the way how applications
> can provide the moment of time at what the packet transmission must be
> started and to describe in preliminary the supporting this feature from mlx5
> PMD side [1].
> 
> The new dynamic timestamp field is proposed, it provides some timing
> information, the units and time references (initial phase) are not explicitly
> defined but are maintained always the same for a given port.
> Some devices allow to query rte_eth_read_clock() that will return the current
> device timestamp. The dynamic timestamp flag tells whether the field
> contains actual timestamp value. For the packets being sent this value can be
> used by PMD to schedule packet sending.
> 
> The device clock is opaque entity, the units and frequency are vendor specific
> and might depend on hardware capabilities and configurations. If might (or
> not) be synchronized with real time via PTP, might (or not) be synchronous
> with CPU clock (for example if NIC and CPU share the same clock source
> there might be no any drift between the NIC and CPU clocks), etc.
> 
> After PKT_RX_TIMESTAMP flag and fixed timestamp field supposed
> deprecation and obsoleting, these dynamic flag and field might be used to
> manage the timestamps on receiving datapath as well. Having the dedicated
> flags for Rx/Tx timestamps allows applications not to perform explicit flags
> reset on forwarding and not to promote received timestamps to the
> transmitting datapath by default.
> The static PKT_RX_TIMESTAMP is considered as candidate to become the
> dynamic flag and this move should be discussed.
> 
> When PMD sees the "rte_dynfield_timestamp" set on the packet being sent it
> tries to synchronize the time of packet appearing on the wire with the
> specified packet timestamp. If the specified one is in the past it should be
> ignored, if one is in the distant future it should be capped with some
> reasonable value (in range of seconds). These specific cases ("too late" and
> "distant future") can be optionally reported via device xstats to assist
> applications to detect the time-related problems.
> 
> There is no any packet reordering according timestamps is supposed, neither
> within packet burst, nor between packets, it is an entirely application
> responsibility to generate packets and its timestamps in desired order. The
> timestamps can be put only in the first packet in the burst providing the
> entire burst scheduling.
> 
> PMD reports the ability to synchronize packet sending on timestamp with
> new offload flag:
> 
> This is palliative and might be replaced with new eth_dev API about
> reporting/managing the supported dynamic flags and its related features.
> This API would break ABI compatibility and can't be introduced at the
> moment, so is postponed to 20.11.
> 
> For testing purposes it is proposed to update testpmd "txonly"
> forwarding mode routine. With this update testpmd application generates
> the packets and sets the dynamic timestamps according to specified time
> pattern if it sees the "rte_dynfield_timestamp" is registered.
> 
> The new testpmd command is proposed to configure sending pattern:
> 
> set tx_times <burst_gap>,<intra_gap>
> 
> <intra_gap> - the delay between the packets within the burst
>               specified in the device clock units. The number
>               of packets in the burst is defined by txburst parameter
> 
> <burst_gap> - the delay between the bursts in the device clock units
> 
> As the result the bursts of packet will be transmitted with specific delays
> between the packets within the burst and specific delay between the bursts.
> The rte_eth_read_clock is supposed to be engaged to get the current device
> clock value and provide the reference for the timestamps.
> 
> [1]
> https://eur03.safelinks.protection.outlook.com/?url=http%3A%2F%2Fpatche
> s.dpdk.org%2Fpatch%2F73714%2F&amp;data=02%7C01%7Cviacheslavo%40
> mellanox.com%7C810609c61c3b466e8f5a08d824ce57f8%7Ca652971c7d2e4
> d9ba6a4d149256f461b%7C0%7C0%7C637299815958194092&amp;sdata=H
> D7efBGOLuYxHd5KLJYJj7RSbiLRVBNm5jdq%2FJv%2FXfk%3D&amp;reserved=
> 0
> 
> Signed-off-by: Viacheslav Ovsiienko <viacheslavo@mellanox.com>

 promote Acked-bt from previous patch version to maintain patchwork 
 status accordingly

Acked-by: Olivier Matz <olivier.matz@6wind.com>

> 
> ---
>   v1->v4:
>      - dedicated dynamic Tx timestamp flag instead of shared with Rx
>   v4->v5:
>      - elaborated commit message
>      - more words about device clocks added,
>      - note about dedicated Rx/Tx timestamp flags added
>   v5->v6:
>      - release notes are updated
>   v6->v7:
>      - commit message is updated
>      - testpmd checks the supported offloads before registering
>        dynamic timestamp flag/field
> ---
>  doc/guides/rel_notes/release_20_08.rst |  7 +++++++
>  lib/librte_ethdev/rte_ethdev.c         |  1 +
>  lib/librte_ethdev/rte_ethdev.h         |  4 ++++
>  lib/librte_mbuf/rte_mbuf_dyn.h         | 31
> +++++++++++++++++++++++++++++++
>  4 files changed, 43 insertions(+)
>