> From: Bruce Richardson [mailto:bruce.richard...@intel.com] > Sent: Thursday, 1 May 2025 09.48 > > On Thu, May 01, 2025 at 09:05:32AM +0200, Morten Brørup wrote: > > > From: Stephen Hemminger [mailto:step...@networkplumber.org] > > > Sent: Wednesday, 30 April 2025 21.45 > > > > > > On Wed, 30 Apr 2025 20:40:52 +0200 > > > Morten Brørup <m...@smartsharesystems.com> wrote: > > > > > > > There are only two thread priorities in the enum > rte_thread_priority: > > > Normal and Real-time Critical. > > > > > > > > I would like to poll ethdev counters, collect garbage and perform > > > other jitter non-sensitive tasks in a control thread with lower > > > priority than my ordinary control threads, so it will be preempted > by > > > any work ready for my ordinary control threads. > > > > > > > > Which DPDK API am I supposed to use to assign this below-normal > > > priority to my "background" control thread? > > > > > > > > Or: Aren't we missing a priority like Linux' SCHED_BATCH? > > > > > > Short answer: if your application is running on Linux, only ever > use > > > Normal. > > > DPDK applications usually never sleep and this will starve the OS > and > > > cause instability. > > > > I was asking for the opposite of Critical priority. > > > > For the sake of discussion, imagine a (registered or unregistered) > non-EAL thread doing something like this: > > loop { > > poll_counters(); // 1 ms execution time > > sleep(99 ms); > > } > > > > With normal scheduling priority, it will rack up a lot of scheduling > credits during sleep(), so it might not be preempted by other threads > while executing poll_counters(). > > > > But if some other thread (on the same CPU core) changes state from > Sleeping to Runnable, I want it to preempt the counter polling thread. > > This other thread could be a control plane application, e.g. a DNS > Server, which shouldn't suffer up to 1 ms scheduling lag if it becomes > Runnable the instant the counter polling thread started executing > poll_counters(). > > > > So I'm looking for a DPDK API to apply a "low priority" scheduling > policy, like SCHED_BATCH, to the counter polling thread. > > > > Does this need to be done in DPDK?
No, not really. > Unless you need to target Windows, would > using the standard Unix/Posix scheduling/pthread APIs directly not be > best, > rather than having us try to wrap all such things inside DPDK APIs? It probably would. That's how we do it today, anyway. :-) > I worry > about scope creep for such things, with us ending up wrapping a whole > bunch > of scheduling stuff into DPDK that we should not need to do. I'm mainly asking for academic reasons. I think the scope of my question was included into DPDK when it introduced the rte_thread_priority with RTE_THREAD_PRIORITY_NORMAL and RTE_THREAD_PRIORITY_REALTIME_CRITICAL. I know this is mainly related to the control plane, and thus not the most relevant thing for DPDK. But I think we need to offer something. Not only for applications, but drivers might want to run separate low-priority threads for background tasks, such as garbage collection, counter polling, or a link state machine. IMHO, the kernel scheduler is a much better choice than DPDK's non-preemptive "Service Cores" scheduler for many purposes.
