----- Original Message -----
> From: [email protected]
> To: "Jan Stancek" <[email protected]>
> Cc: [email protected]
> Sent: Thursday, 3 October, 2013 3:13:06 PM
> Subject: Re: [LTP] [PATCH 3/3] pthread_attr_setschedpolicy/2-1.c: give
> threads a moment to block on mutex
>
> Hi!
> > > Hmm, so you did hit the small window for race condition between the new
> > > thread signals the main thread that it's executed and the next call to
> > > the mutex_lock on the tested mutex?
> >
> > Yes, I'm assuming this is reason why I see it sporadically failing on
> > different distros/kernels. I see it happening the most on IBM z/VM (s390)
> > guests which have overcommitted resources, which affects scheduling.
> >
> > It takes longer, but I could trigger it on x86_64 too.
>
> I think that I know where the problem may come from. We are starting the
> threads in reverse order by priority, so the lowest one can actually be
> preempted by the higher priority one before the lock is taken, which
> makes sense.
>
> > > I do not like this solution much, but this is not easy to do properly.
> > > One posibility is to pinpoint the threads on one cpu via the affinity()
> > > interface (open_posix_testsuite/include/affinity.h) then we can wait in
> > > the main thread until the thread with lowest priority is executed and
> > > safely say that the rest is locked on the mutex allready (as they run
> > > with FIFO scheduling).
> >
> > I was considering this too, but there are some problems:
> >
> > 1. pthread_setaffinity_np is not portable / sched_setaffinity is Linux only
>
> This is the reason we have the open_posix_testsuite/include/affinity.h,
> it's not portable but some test cannot be written without it and it's
> implemented on most POSIX OSes, so we keep the unportable chunks in one
> place.
>
> > 2. linux default sched_rt_runtime_us gives 0.05s to be used by
> > SCHED_OTHER (non-RT tasks) (Documentation/scheduler/sched-rt-group.txt)
> > If we set also main thread to use SCHED_FIFO, then this shouldn't be
> > problem.
>
> That shouldn't be much of problem, the main thread would pick up once
> the realtime threads are locked on mutexes/cond vars. Or am I mistaken
> here?
(assuming all threads are bound to single CPU)
That's true, but (the way I understand it) non-rt main thread can also run
when scheduler thinks that RT processes are monopolizing CPU for too long.
So you can't be sure that thread is actually on "mutex".
thread_func():
pthread_mutex_lock(&c_mutex);
pthread_cond_signal(&cond);
pthread_mutex_unlock(&c_mutex);
--> here scheduler decides it's time to give a chance to non-rt threads
and runs main thread, condition was already signalled so main thinks
thread is already blocking on mutex
pthread_mutex_lock(&mutex);
If we set SCHED_FIFO also on main thread with lowest priority, then
it shouldn't run regardless of value in sched_rt_runtime_us.
Regards,
Jan
>
> --
> Cyril Hrubis
> [email protected]
>
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