Re: [PATCH] sched/rt: Fix live lock between select_fallback_rq() and RT push

[Joel Fernandes]

Hi Steve,

On Fri, Sep 22, 2023 at 09:45:39PM -0400, Steven Rostedt wrote:
> On Sat, 23 Sep 2023 01:14:08 +0000
> "Joel Fernandes (Google)" <joel@xxxxxxxxxxxxxxxxx> wrote:
> 
> > During RCU-boost testing with the TREE03 rcutorture config, I found that
> > after a few hours, the machine locks up.
> > 
> > On tracing, I found that there is a live lock happening between 2 CPUs.
> > One CPU has an RT task running, while another CPU is being offlined
> > which also has an RT task running.  During this offlining, all threads
> > are migrated. The migration thread is repeatedly scheduled to migrate
> > actively running tasks on the CPU being offlined. This results in a live
> > lock because select_fallback_rq() keeps picking the CPU that an RT task
> > is already running on only to get pushed back to the CPU being offlined.
> > 
> > It is anyway pointless to pick CPUs for pushing tasks to if they are
> > being offlined only to get migrated away to somewhere else. This could
> > also add unwanted latency to this task.
> > 
> > Fix these issues by not selecting CPUs in RT if they are not 'active'
> > for scheduling, using the cpu_active_mask. Other parts in core.c already
> > use cpu_active_mask to prevent tasks from being put on CPUs going
> > offline.
> > 
> > Tested-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> > Cc: stable@xxxxxxxxxxxxxxx
> > Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> > ---
> >  kernel/sched/cpupri.c | 1 +
> >  1 file changed, 1 insertion(+)
> > 
> > diff --git a/kernel/sched/cpupri.c b/kernel/sched/cpupri.c
> > index a286e726eb4b..42c40cfdf836 100644
> > --- a/kernel/sched/cpupri.c
> > +++ b/kernel/sched/cpupri.c
> > @@ -101,6 +101,7 @@ static inline int __cpupri_find(struct cpupri *cp, struct task_struct *p,
> >  
> >  	if (lowest_mask) {
> >  		cpumask_and(lowest_mask, &p->cpus_mask, vec->mask);
> > +		cpumask_and(lowest_mask, lowest_mask, cpu_active_mask);
> 
> What happens if the cpu_active_mask changes right here?
> 
> Is this just making the race window smaller?

It should not be an issue for fixing the live lock because at most that would
cause a few more bounces between the 2 CPUs but eventually once
cpu_active_mask is stable, the CPU being offlined will not be selected for
the push. That's nothing compared to the multi-second live lock that happens
right now.

Also, with this patch I ran the tests for days and could not reproduce the
issue. Without the patch, I hit it in a few hours.

> Something tells me the fix is going to be something a bit more involved.
> But as I'm getting ready for Paris, I can't look at it at the moment.

Thanks for taking a look and safe travels!

 - Joel