On Tue, 15 Jul 2025 16:15:22 +0000 Scott Wasson <swas...@microsoft.com> 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. 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. > > 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. > > 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. > > Thanks in advance for your help! > > -Scott > There is no locking in driver path for control. It is expected that application will manage access to control path (RSS being one example) so that only one thread modifies the PMD.
