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From: Thomas Monjalon <thomas@monjalon.net>
To: Morten =?ISO-8859-1?Q?Br=F8rup?= <mb@smartsharesystems.com>,
 Stephen Hemminger <stephen@networkplumber.org>,
 Min Zhou <zhoumin@loongson.cn>
Cc: stable@dpdk.org, dev@dpdk.org, David Marchand <david.marchand@redhat.com>, 
 stable@dpdk.org, Anatoly Burakov <anatoly.burakov@intel.com>,
 Narcisa Vasile <navasile@linux.microsoft.com>,
 Tyler Retzlaff <roretzla@linux.microsoft.com>,
 Dmitry Kozlyuk <dmitry.kozliuk@gmail.com>,
 Konstantin Ananyev <konstantin.v.ananyev@yandex.ru>,
 Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
Subject: Re: [PATCH] eal/unix: allow creating thread with real-time priority
Date: Wed, 25 Oct 2023 15:15:49 +0200
Message-ID: <3767229.kQq0lBPeGt@thomas>
In-Reply-To: <20231024090450.519ac82b@hermes.local>
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Errors-To: stable-bounces@dpdk.org

24/10/2023 18:04, Stephen Hemminger:
> On Tue, 24 Oct 2023 15:55:13 +0200
> Morten Br=F8rup <mb@smartsharesystems.com> wrote:
>=20
> > >=20
> > >    4. It MAY be used by preemptible multi-producer and/or preemptible=
 multi-
> > > consumer pthreads whose scheduling policy are all SCHED_OTHER(cfs), S=
CHED_IDLE
> > > or SCHED_BATCH. User SHOULD be aware of the performance penalty befor=
e using
> > > it.
> > >=20
> > > -  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``). =20
> >=20
> > Do the RTS or HTS ring modes make any difference here?
> >=20
> > Anyway, I agree that real-time priority should not be forbidden on Unix.
> >=20
> > Acked-by: Morten Br=F8rup <mb@smartsharesystems.com>
>=20
> 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 do=
ne
> in DPDK applications. A real time thread has higher priority than other n=
ecessary
> kernel threads on the same CPU. Therefore if the RT thread never sleeps, =
critical
> system actions such as delayed writes, network packet processing and time=
r 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.