Re: [PATCH] eal/unix: allow creating thread with real-time priority

[Thomas Monjalon <thomas@monjalon.net>]

24/10/2023 18:04, Stephen Hemminger:
> On Tue, 24 Oct 2023 15:55:13 +0200
> Morten Brørup <mb@smartsharesystems.com> wrote:
> 
> > > 
> > >    4. It MAY be used by preemptible multi-producer and/or preemptible multi-
> > > consumer pthreads whose scheduling policy are all SCHED_OTHER(cfs), SCHED_IDLE
> > > or SCHED_BATCH. User SHOULD be aware of the performance penalty before using
> > > it.
> > > 
> > > -  5. It MUST not be used by multi-producer/consumer pthreads, whose
> > > scheduling policies are SCHED_FIFO or SCHED_RR.
> > > +  5. It MUST not be used by multi-producer/consumer pthreads
> > > +     whose scheduling policies are ``SCHED_FIFO``
> > > +     or ``SCHED_RR`` (``RTE_THREAD_PRIORITY_REALTIME_CRITICAL``).  
> > 
> > Do the RTS or HTS ring modes make any difference here?
> > 
> > Anyway, I agree that real-time priority should not be forbidden on Unix.
> > 
> > Acked-by: Morten Brørup <mb@smartsharesystems.com>
> 
> Please add a big warning message in the rte_thread.c and the documentation
> to describe the problem. Need to have the "you have been warned" action.

Yes I can add more warnings.

> Use of RT priority is incompatible with 100% poll mode as is typically done
> in DPDK applications. A real time thread has higher priority than other necessary
> kernel threads on the same CPU. Therefore if the RT thread never sleeps, critical
> system actions such as delayed writes, network packet processing and timer updates
> will not happen which makes the system unstable.

Yes, and it is shown by the test on loongarch:
DPDK:fast-tests / threads_autotest        TIMEOUT        80.01s
http://mails.dpdk.org/archives/test-report/2023-October/488760.html

I'll try to pass the test by adding a sleep in the test thread.

> Multiple DPDK users have learned this the hard way.