Re: [dpdk-dev] [PATCH] ethdev: rte_eth_rx_burst()nb_pktsrequirements

[Morten Brørup <mb@smartsharesystems.com>]

> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Ferruh Yigit
> Sent: Tuesday, October 13, 2020 12:56 PM
> 
> On 9/14/2020 1:42 PM, Morten Brørup wrote:
> >> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Bruce Richardson
> >> Sent: Monday, September 14, 2020 1:27 PM
> >>
> >> On Mon, Sep 14, 2020 at 01:05:11PM +0200, Morten Brørup wrote:
> >>> Updated description of rte_eth_rx_burst() to reflect what drivers,
> >>> when using vector instructions, expect from nb_pkts.
> >>>
> >>> Also discussed on the mailing list here:
> >>>
> >> http://inbox.dpdk.org/dev/98CBD80474FA8B44BF855DF32C47DC35C61257@smarts
> >> erver.smartshare.dk/
> >>>
> >>> Signed-off-by: Morten Brørup <mb@smartsharesystems.com>
> >>> ---
> >>>   lib/librte_ethdev/rte_ethdev.h | 5 +++++
> >>>   1 file changed, 5 insertions(+)
> >>>
> >>> diff --git a/lib/librte_ethdev/rte_ethdev.h
> >> b/lib/librte_ethdev/rte_ethdev.h
> >>> index 70295d7ab..41f8ba4ef 100644
> >>> --- a/lib/librte_ethdev/rte_ethdev.h
> >>> +++ b/lib/librte_ethdev/rte_ethdev.h
> >>> @@ -4469,6 +4469,10 @@ int
> >> rte_eth_dev_hairpin_capability_get(uint16_t port_id,
> >>>    * burst-oriented optimizations in both synchronous and asynchronous
> >>>    * packet processing environments with no overhead in both cases.
> >>>    *
> >>> + * @note
> >>> + *   Some drivers using vector instructions require that *nb_pkts*
> >> is
> >>> + *   divisible by 4 or 8, depending on the driver implementation.
> >>> + *
> >>
> >> Not technically true, in that the drivers will round the value down to
> >> the
> >> nearest multiple of 4 or 8. So how about rewording as:
> >>
> >> "Some drivers using vector instructions may round the *nb_pkts* driver
> >> to
> >> a multiple of 4 or 8 depending upon the driver implementation."
> >>
> >
> > You are correct about the driver behavior.
> >
> > However, if you pass nb_pkts=9, the driver will return 8 packets,
> > and thus it does not conform to the API behavior of returning nb_pkts
> > if they are there.
> >
> > This is why the description in this patch differs from the description we
> reached in the RFC discussion.
> >
> 
> Hi Morten, Bruce,
> 
> +1 to document the this behavior.
> 
> But in the patch the wording is more strict:
> "... require that *nb_pkts* is divisible by 4 or 8 ..."
> "... The value must be divisible by 8 in order to work with any driver."
> 
> I am not sure the requirement is that strict. Application still provide any
> value for 'nb_pkts', so the value doesn't "have to" be divisible 8/4.
> 
> But for vector PMD case it will return number of packets round down to 8/4.
> Perhaps can add for vector PMD it must be at least 4/8?
> 
> Bruce's explanation sound more accurate to me, what do you think?
> 

I aim to keep the explanation in the documentation relatively simple. Keep the parameter description short, and add the details about vector driver behavior as a note to the function.

The reason for all this is the existing documentation describing how to use the rte_eth_rx_burst() function at high level:

The rte_eth_rx_burst() function returns the number of packets actually retrieved [...]. A return value equal to nb_pkts indicates [...] that other received packets remain in the input queue. Applications implementing a "retrieve as much received packets as possible" policy can check this specific case and keep invoking the rte_eth_rx_burst() function until a value less than nb_pkts is returned.

As an alternative to my proposed solution, we could add that vector drivers round down to 4 or 8, and the application's comparison of the nb_pkts and return value must consider this. But that would make the above description strangely complex, rather than just requiring that nb_pkts for vector drivers must be divisible by 4 or 8.

And as a minor detail, keeping my proposed restriction would also eliminate the vector drivers' need to round down.

I don't see a need to be able to call rte_eth_rx_burst() with a value not divisible by 4 or 8 for a vector driver, so my proposed restriction is a tradeoff favoring simplicity over unnecessary flexibility.

> 
> >>>    * The rte_eth_rx_burst() function does not provide any error
> >>>    * notification to avoid the corresponding overhead. As a hint, the
> >>>    * upper-level application might check the status of the device link
> >> once
> >>> @@ -4485,6 +4489,7 @@ int rte_eth_dev_hairpin_capability_get(uint16_t
> >> port_id,
> >>>    *   must be large enough to store *nb_pkts* pointers in it.
> >>>    * @param nb_pkts
> >>>    *   The maximum number of packets to retrieve.
> >>> + *   The value must be divisible by 8 in order to work with any
> >> driver.
> >>
> >> Similarly here, I think it's better to state that it should be at least
> >> 8,
> >> and any values not divisible by 8 may be rounded down.
> >>
> >>>    * @return
> >>>    *   The number of packets actually retrieved, which is the number
> >>>    *   of pointers to *rte_mbuf* structures effectively supplied to
> >> the
> >>> --
> >>> 2.17.1
> >>>
> >
>