Re: INFO: rcu_sched detected stalls on CPUs/tasks with `kswapd` and `mem_cgroup_shrink_node`

[Paul E. McKenney]

On Wed, Nov 30, 2016 at 12:09:44PM +0100, Michal Hocko wrote:
> [CCing Paul]
> 
> On Wed 30-11-16 11:28:34, Donald Buczek wrote:
> [...]
> > shrink_active_list gets and releases the spinlock and calls cond_resched().
> > This should give other tasks a chance to run. Just as an experiment, I'm
> > trying
> > 
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -1921,7 +1921,7 @@ static void shrink_active_list(unsigned long
> > nr_to_scan,
> >         spin_unlock_irq(&pgdat->lru_lock);
> > 
> >         while (!list_empty(&l_hold)) {
> > -               cond_resched();
> > +               cond_resched_rcu_qs();
> >                 page = lru_to_page(&l_hold);
> >                 list_del(&page->lru);
> > 
> > and didn't hit a rcu_sched warning for >21 hours uptime now. We'll see.
> 
> This is really interesting! Is it possible that the RCU stall detector
> is somehow confused?

No, it is not confused.  Again, cond_resched() is not a quiescent
state unless it does a context switch.  Therefore, if the task running
in that loop was the only runnable task on its CPU, cond_resched()
would -never- provide RCU with a quiescent state.

In contrast, cond_resched_rcu_qs() unconditionally provides RCU
with a quiescent state (hence the _rcu_qs in its name), regardless
of whether or not a context switch happens.

It is therefore expected behavior that this change might prevent
RCU CPU stall warnings.

							Thanx, Paul

> > Is preemption disabled for another reason?
> 
> I do not think so. I will have to double check the code but this is a
> standard sleepable context. Just wondering what is the PREEMPT
> configuration here?
> -- 
> Michal Hocko
> SUSE Labs
>