Hi Scott,

On Tue, 15 Jul 2025, Scott Wasson wrote:

> 
> Hi,
> 
>  
> 
> We’re using multiqueue, and RSS doesn’t always balance the load very well.  I had a clever idea to periodically measure the load distribution (cpu load on the IO cores)  in the
> background pthread, and use rte_eth_dev_rss_reta_update() to adjust the redirection table dynamically if the imbalance exceeds a given threshold.  In practice it seems to work nicely.

As far as I remember, there has already been an academic project [1] that would
do (almost) the same thing: dynamically reprogram the table based on the current
load status. I vaguely remember mentions of DPDK RETA update in video [2], so
perhaps Tom can shed some light on how the API is invoked lock-wise (Cc Tom).

[1] https://dejankosticgithub.github.io/documents/publications/rsspp-conext19.pdf
[2] https://www.youtube.com/watch?v=YV3aJOxjUqI

>   But I’m concerned about:
> 
> https://doc.dpdk.org/api/rte__ethdev_8h.html#a3c1540852c9cf1e576a883902c2e310d 
> 
> Which states:
> 
> 
> By default, all the functions of the Ethernet Device API exported by a PMD are lock-free functions which assume to not be invoked in parallel on different logical cores to work on the
> same target object. For instance, the receive function of a PMD cannot be invoked in parallel on two logical cores to poll the same Rx queue [of the same port]. Of course, this function
> can be invoked in parallel by different logical cores on different Rx queues. It is the responsibility of the upper level application to enforce this rule.
> 
>  
> 
> In this context, what is the “target object”?  The queue_id of the port?  Or the port itself?  Would I need to add port-level spinlocks around every invocation of rte_eth_dev_*()? 
> That’s a hard no, it would destroy performance.

My guess is that it mostly refers to the queue receive/transmit operations. So
the target object might be the DMA queue (Rx/Tx) of the device in question. As
for the control-plane APIs in 'ethdev', I would imagine the typical usage of
these was meant to be via a dedicated control-plane core. So if one can gather
load statistics from IO workers in some clever message-oriented way so that the
main (control) lcore can read those periodically and invoke RETA update, that
would ideally not create contention on the control-plane lock of the port.

More to that, in particular, for the RETA update API, I guess many drivers have
an implicit/internal port lock in place, so it is going to be leveraged anyway.

> 
>  
> 
> Alternatively, if I were to periodically call rte_eth_dev_rss_reta_update() from the IO cores instead of the background core, as the above paragraph suggests, that doesn’t seem correct
> either.  The function takes a reta_conf[] array that affects all RETA entries for that port and maps them to a queue_id.  Is it safe to remap RETA entries for a given port on one IO core
> while another IO core is potentially reading from its rx queue for that same port?  That problem seems not much different from remapping in the background core as I am now.

That may not be desirable for the reason explained above: implicit port locks
in vendor-specific implementations of the control-plane APIs like RETA update.

Regarding the question on whether it is safe to reprogram RETA "on the fly" (we
may assume this is done from the main/control lcore): it was my impression that
doing so would affect some HW component sitting deep in the NIC that distributes
packets across DMA queues (which are one level above), not the queues per se,
which just contain packets that have been distributed so far. Reprogramming RSS
in general might not have that much to do with DMA queues, if I'm not mistaken.

> 
>  
> 
> I’m starting to suspect this function was intended to be initialized once on startup before rte_eth_dev_start(), and/or the ports must be stopped before calling it.  If that’s the case,
> then I’ll call this idea too clever by half and give it up now.

Not really. There are PMDs that support RETA update in the started state. It
should be fine to invoke this API from a single control-plane core. Why not?
If the device does not support it in started state, it is the duty of the driver
to either remember the table to be set on next port start or return an error.

Also, while RETA update API of 'ethdev' is meant to control the "global" RSS
setting, there is also RTE flow API's action 'RSS' [3] that can be shared among
multiple flow rule objects by means of a container action, 'INDIRECT', [4]. For
example, one can create a flow rule generic enough to target a wide subset of
flows (or multiple flow rules targeting the same 'INDIRECT' RSS action) and then
use an update API [5] to update specifically this shared action. That being
said, once again, this update is better be done from some central place (main
lcore), as even if the PMD says it supports thread-safe flow API (by setting
flag [6]), this may just mean that the driver uses locks internally. However,
there might be vendors who support so-called "asynchronous flow API", which uses
slightly different flow management APIs ([7], for instance), but those are
clearly designed to be invoked by IO workers when they see interesting traffic
and either need to insert new flows or update some shared actions "on the fly".

[3] https://doc.dpdk.org/api-25.03/rte__flow_8h.html#a78f0386e683cfc491462a771df8b971aa72428c7c1896fe4dfdc2dbed85214d27
[4] https://doc.dpdk.org/api-25.03/rte__flow_8h.html#a78f0386e683cfc491462a771df8b971aa47ea41707def29ff416e233434ab33a6
[5] https://doc.dpdk.org/api-25.03/rte__flow_8h.html#aea5b96385043898923f3b1690a72d2c0
[6] https://doc.dpdk.org/api-25.03/rte__ethdev_8h.html#a3c1540852c9cf1e576a883902c2e310d
[7] https://doc.dpdk.org/api-25.03/rte__flow_8h.html#a5097c64396d74102d8f2ae119c9dc7d5

Thank you.

> 
>  
> 
> Thanks in advance for your help!
> 
>  
> 
> -Scott
> 
>  
> 
> 
>