Re: [dpdk-dev] [EXT] Re: [PATCH 1/3] mbuf: add Tx offloads for packet marking

[Olivier Matz <olivier.matz@6wind.com>]

Hi Nithin,

On Wed, Jun 03, 2020 at 04:14:14PM +0530, Nithin Dabilpuram wrote:
> On Wed, Jun 03, 2020 at 10:28:44AM +0200, Olivier Matz wrote:
> > Hi,
> > 
> > On Tue, Jun 02, 2020 at 07:55:37PM +0530, Nithin Dabilpuram wrote:
> > > On Tue, Jun 02, 2020 at 10:53:08AM +0000, Ananyev, Konstantin wrote:
> > > > Hi Jerin,
> > > > 
> > > > > > > > I also share Olivier's concern about consuming 3 bits in ol_flags for that feature.
> > > > > > > > Can it probably be squeezed somehow?
> > > > > > > > Let say we reserve one flag that this information is present or not, and
> > > > > > > > re-use one of rx-only fields for store additional information (packet_type, or so).
> > > > > > > > Or might be some other approach.
> > > > > > >
> > > > > > > We are fine with this approach where we define one bit in Tx offloads for pkt
> > > > > > > marking and and 3 bits reused from Rx offload flags area.
> > > > > > >
> > > > > > > For example:
> > > > > > >
> > > > > > > @@ -186,10 +186,16 @@ extern "C" {
> > > > > > >
> > > > > > >  /* add new RX flags here, don't forget to update PKT_FIRST_FREE */
> > > > > > >
> > > > > > > +/* Reused Rx offload bits for Tx offloads */
> > > > > > > +#define PKT_X_TX_MARK_VLAN_DEI         (1ULL << 0)
> > > > > > > +#define PKT_X_TX_MARK_IP_DSCP          (1ULL << 1)
> > > > > > > +#define PKT_X_TX_MARK_IP_ECN           (1ULL << 2)
> > > > > > > +
> > > > > > >  #define PKT_FIRST_FREE (1ULL << 23)
> > > > > > > -#define PKT_LAST_FREE (1ULL << 40)
> > > > > > > +#define PKT_LAST_FREE (1ULL << 39)
> > > > > > >
> > > > > > >  /* add new TX flags here, don't forget to update PKT_LAST_FREE  */
> > > > > > > +#define PKT_TX_MARK_EN         (1ULL << 40)
> > > > > > >
> > > > > > > Is this fine ?
> > > > > >
> > > > > > Any thoughts on this approach which uses only 1 bit in Tx flags out of 18
> > > > > > and reuse unused Rx flag bits ?
> > > > 
> > > > My thought was not about re-defining the flags (I think it is better to keep them intact),
> > > > but adding a union for one of rx-only fields (packet_type/rss/timestamp).
> > > 
> > > Ok. Adding a union field at packet_type field is also fine like below. 
> > > 
> > > @@ -187,9 +187,10 @@ extern "C" {
> > >  /* add new RX flags here, don't forget to update PKT_FIRST_FREE */
> > >  
> > >  #define PKT_FIRST_FREE (1ULL << 23)
> > > -#define PKT_LAST_FREE (1ULL << 40)
> > > +#define PKT_LAST_FREE (1ULL << 39)
> > >  
> > >  /* add new TX flags here, don't forget to update PKT_LAST_FREE  */
> > > +#define PKT_TX_MARK_EN		(1ULL << 40)
> > >  
> > >  /**
> > >   * Outer UDP checksum offload flag. This flag is used for enabling
> > > @@ -461,6 +462,14 @@ enum {
> > >  #endif
> > >  };
> > >  
> > > +/* Tx packet marking flags in rte_mbuf::tx_mark.
> > > + * Valid only when PKT_TX_MARK_EN is set in
> > > + * rte_mbuf::ol_flags.
> > > + */
> > > +#define TX_MARK_VLAN_DEI	(1ULL << 0)
> > > +#define TX_MARK_IP_DSCP	(1ULL << 1)
> > > +#define TX_MARK_IP_ECN		(1ULL << 2)
> > > +
> > >  /**
> > >   * The generic rte_mbuf, containing a packet mbuf.
> > >   */
> > > @@ -543,6 +552,10 @@ struct rte_mbuf {
> > >  			};
> > >  			uint32_t inner_l4_type:4; /**< Inner L4 type. */
> > >  		};
> > > +		struct {
> > > +			uint32_t reserved:29;
> > > +			uint32_t tx_mark:3;
> > > +		};
> > >  	};
> > > 
> > > 
> > > 
> > > Please correct me if this is not what you mean.
> > 
> > I'm not a big fan of reusing Rx fields or flags for Tx.
> > It's not obvious for an application than adding a tx_mark will overwrite
> > the packet_type. I understand that the risk is limited because packet_type
> > is Rx and the marks are Tx, but there is still one.
> 
> I'm also not a big fan but just wanted to take this approach so that,
> it can both conserve space and also help fast path.
> Reusing Rx area is however not a new thing as is already followed for
> mbuf->txadapter field.

Yes, and in my opinion this is something we should avoid when possible,
because it makes some features exclusive (ex: the big union with
sched/rss/adapter/usr/...).

> Apart from documentation issue, Is there any other issue or future 
> ramification with using Rx field's for Tx ?

No, I don't see any other issue except the ones we already mentioned (doc, code clarity, ).

> If it is only about documentation, then we can add more documentation to make things clear.
> > 
> > To summarize the different proposed approaches (please correct me if I'm wrong):
> > 
> > a- add 3 Tx mbuf flags
> >    (-) consumes limited resource
> > 
> > b- add 3 dynamic flags
> >    (-) slower
> 
> - Tx burst Vector implementation can't be done for this tx offload as
>   offset keeps changing.

A vector implementation can be done. But yes, it would be slower than
with a static flag.

> > 
> > c- add 1 Tx flag and union with Rx field
> >    (-) exclusive with Rx field
> >    (-) still consumes one flag
> > 
> > My preference is still b-, for these reasons:
> > 
> > - There are many different DPDK use cases, and resources in mbuf is tight.
> >   Recent contributions (rte_flow and ice driver) already made use of dynamic
> >   fields/flags.
> - Since RTE_FLOW metadata is 32-bit field, it is a clear candidate for
> dynamic flags.

I'm not sure to get why it is a better candidate than packet marking.
You mean because it requires more room in mbuf?

> - ICE PMD's dynamic field is however a vendor specific field and only for
> ICE PMD users.

Yes, but ICE PMD users may be as important as packet marking users.

> In this case, it is just 1 bit out of 18 free bits available in ol_flags.
> 
> > 
> > - When I implemented the dynamic fields/flags feature, I did a test which
> >   showed that the cost of having a dynamic offset was few cycles (on my test
> >   platform, it was~3 cycles for reading a field and ~2 cycles for writing a
> >   field).
> 
> I think this cost is of the case where the address where the dyn_offset is
> stored is already in cache as it needs to be read first.

This fetch of the value (in case it is not in cache) can be done once per bulk,
so I'm not sure the impact would be high.


Regards,
Olivier


> 
> 
> > 
> > Regards,
> > Olivier
> > 
> > > 
> > > > 
> > > > > 
> > > > > + Techboard
> > > > > 
> > > > > There is a related thread going on
> > > > > https://urldefense.proofpoint.com/v2/url?u=http-3A__mails.dpdk.org_archives_dev_2020-2DMay_168810.html&d=DwIGaQ&c=nKjWec2b6R0mOyPaz7xtfQ&r=FZ_tPCbgFOh18zwRPO9H0yDx8VW38vuapifdDfc8SFQ&m=nyV4Rud03HW6DbWMpyvOCulQNkagmfo0wKtrwQ7zmmg&s=VuktoUb_xoLsHKdB9mV87x67cP9tXk3DqVXptt9nF_s&e= 
> > > > > 
> > > > > If there is no consensus on email, then I would like to add this item
> > > > > to the next TB meeting.
> > > > 
> > > > Ok, I'll add that to tomorrow meeting agenda.
> > > > Konstantin
> > > > 
> > 
> >