Re: [dpdk-dev] [BUG] service_lcore_en_dis_able from service_autotest failing

["Van Haaren, Harry" <harry.van.haaren@intel.com>]

> -----Original Message-----
> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Van Haaren, Harry
> Sent: Monday, October 14, 2019 5:49 PM
> To: Aaron Conole <aconole@redhat.com>
> Cc: David Marchand <david.marchand@redhat.com>; dev@dpdk.org
> Subject: Re: [dpdk-dev] [BUG] service_lcore_en_dis_able from service_autotest
> failing
> 
> > -----Original Message-----
> > From: Aaron Conole [mailto:aconole@redhat.com]
> > Sent: Monday, October 14, 2019 3:54 PM
> > To: Van Haaren, Harry <harry.van.haaren@intel.com>
> > Cc: David Marchand <david.marchand@redhat.com>; dev@dpdk.org
> > Subject: Re: [dpdk-dev] [BUG] service_lcore_en_dis_able from
> > service_autotest failing
> >
> > Aaron Conole <aconole@redhat.com> writes:
> >
> > > "Van Haaren, Harry" <harry.van.haaren@intel.com> writes:
> > >
> > >>> -----Original Message-----
> > >>> From: Aaron Conole [mailto:aconole@redhat.com]
> > >>> Sent: Wednesday, September 4, 2019 8:56 PM
> > >>> To: David Marchand <david.marchand@redhat.com>
> > >>> Cc: Van Haaren, Harry <harry.van.haaren@intel.com>; dev@dpdk.org
> > >>> Subject: Re: [dpdk-dev] [BUG] service_lcore_en_dis_able from
> > service_autotest
> > >>> failing
> <snip lots of backlog>
> > >>> > real    2m42.884s
> > >>> > user    5m1.902s
> > >>> > sys    0m2.208s
> > >>>
> > >>> I can confirm - takes about 1m to fail.
> > >>
> > >>
> > >> Hi Aaron and David,
> > >>
> > >> I've been attempting to reproduce this, still no errors here.
> > >>
> > >> Given the nature of service-cores, and the difficulty to reproduce
> > >> here this feels like a race-condition - one that may not exist in all
> > >> binaries. Can you describe your compiler/command setup? (gcc 7.4.0 here).
> > >>
> > >> I'm using Meson to build, so reproducing using this instead of the
> > command
> > >> as provided above. There should be no difference in reproducing due to
> > this:
> > >
> > > The command runs far more iterations than meson does (I think).
> > >
> > > I still see it periodically occur in the travis environment.
> > >
> > > I did see at least one missing memory barrier (I believe).  Please
> > > review the following code change (and if you agree I can submit it
> > > formally):
> > >
> > > -----
> > > --- a/lib/librte_eal/common/eal_common_launch.c
> > > +++ b/lib/librte_eal/common/eal_common_launch.c
> > > @@ -21,8 +21,10 @@
> > >  int
> > >  rte_eal_wait_lcore(unsigned slave_id)
> > >  {
> > > -       if (lcore_config[slave_id].state == WAIT)
> > > +       if (lcore_config[slave_id].state == WAIT) {
> > > +               rte_rmb();
> > >                 return 0;
> > > +       }
> > >
> > >         while (lcore_config[slave_id].state != WAIT &&
> > >                lcore_config[slave_id].state != FINISHED)
> > > -----
> > >
> > > This is because in lib/librte_eal/linux/eal/eal_thread.c:
> > >
> > > -----
> > > 		/* when a service core returns, it should go directly to WAIT
> > > 		 * state, because the application will not lcore_wait() for it.
> > > 		 */
> > > 		if (lcore_config[lcore_id].core_role == ROLE_SERVICE)
> > > 			lcore_config[lcore_id].state = WAIT;
> > > 		else
> > > 			lcore_config[lcore_id].state = FINISHED;
> > > -----
> > >
> > > NOTE that the service core skips the rte_eal_wait_lcore() code from
> > > making the FINISHED->WAIT transition.  So I think at least that read
> > > barrier will be needed (maybe I miss the pairing, though?).
> > >
> > > Additionally, I'm wondering if there is an additional write or sync
> > > barrier needed to ensure that some of the transitions are properly
> > > recorded when using lcore as a service lcore function.  The fact that
> > > this only happens occasionally tells me that it's either a race (which
> > > is possible... because the variable update in the test might not be
> > > sync'd across cores or something), or some other missing
> > > synchronization.
> > >
> > >> $ meson test service_autotest --repeat 50
> > >>
> > >> 1/1 DPDK:fast-tests / service_autotest      OK       3.86 s
> > >> 1/1 DPDK:fast-tests / service_autotest      OK       3.87 s
> > >> ...
> > >> 1/1 DPDK:fast-tests / service_autotest      OK       3.84 s
> > >>
> > >> OK:        50
> > >> FAIL:       0
> > >> SKIP:       0
> > >> TIMEOUT:    0
> > >>
> > >> I'll keep it running for a few hours but I have little faith if it only
> > >> takes 1 minute on your machines...
> > >
> > > Please try the flat command.
> >
> > Not sure if you've had any time to look at this.
> 
> Apologies for delay in response - I've ran the existing tests a few 1000's of
> times during the week, with one reproduction. That's not enough for confidence
> in debug/fix for me.
> 
> 
> > I think there's a change we can make, but not sure about how it fits in
> > the overall service lcore design.
> 
> This suggestion is only changing the test code correct?
> 
> 
> > The proposal is to use a pthread_cond variable which blocks the thread
> > requesting the service function to run.  The service function merely
> > sets the condition.  The requesting thread does a timed wait (up to 5s?)
> > and if the timeout is exceeded can throw an error.  Otherwise, it will
> > unblock and can assume that the test passes.  WDYT?  I think it works
> > better than the racy code in the test case for now.
> 
> The idea/concept is right above, but I think that's what the test is
> approximating anyway? The main thread does an "mp_wait_lcore()" until
> the service core has returned, essentially a blocking call.
> 
> The test fails if the flag is not == 1 (as that indidcates failure in
> launching
> an application function on a previously-use-as-service-core lthread).
> 
> I think your RMB suggestion is likely to be the correct, but I'd like to dig
> into it a bit more.
> 
> Thanks for the ping on this thread.

Good news - adding a rte_delay_ms() to the start of service_remote_launch_func()
makes this 100% (so far) reproducible here. So yes it's a race condition,
and I think I have a handle on why/what - it's an (lcore.state == WAIT) race.

To be continued...