Re: [LTP] [PATCH 3/3] pthread_attr_setschedpolicy/2-1.c: give threads a moment to block on mutex

[chrubis@suse.cz]

Hi!
> > > 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.

As I understand it, it's a anti lockup measure so if the kernel detects
that realtime threads has been running for nearly a second in a row it
stops the execution and schedules some other threads. But that shouldn't
be the case here.

> 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.

That is why I said that we have to wait for the lowest priority thread
to run. Giving the main thread even lower priority is a way of doing
that.

-- 
Cyril Hrubis
chrubis@suse.cz

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