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

[Paul E. McKenney]

On Mon, Nov 21, 2016 at 03:18:19PM +0100, Michal Hocko wrote:
> On Mon 21-11-16 06:01:22, Paul E. McKenney wrote:
> > On Mon, Nov 21, 2016 at 02:41:31PM +0100, Michal Hocko wrote:
> [...]
> > > To the patch. I cannot say I would like it. cond_resched_rcu_qs sounds
> > > way too lowlevel for this usage. If anything cond_resched somewhere inside
> > > mem_cgroup_iter would be more appropriate to me.
> > 
> > Like this?
> > 
> > 							Thanx, Paul
> > 
> > ------------------------------------------------------------------------
> > 
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index ae052b5e3315..81cb30d5b2fc 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -867,6 +867,7 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
> >  out:
> >  	if (prev && prev != root)
> >  		css_put(&prev->css);
> > +	cond_resched_rcu_qs();
> 
> I still do not understand why should we play with _rcu_qs at all and a
> regular cond_resched is not sufficient. Anyway I would have to double
> check whether we can do cond_resched in the iterator. I do not remember
> having users which are atomic but I might be easily wrong here. Before
> we touch this code, though, I would really like to understand what is
> actually going on here because as I've already pointed out we should
> have some resched points in the reclaim path.

If there is a tight loop in the kernel, cond_resched() will ensure that
other tasks get a chance to run, but if there are no such tasks, it does
nothing to give RCU the quiescent state that it needs from time to time.
So if there is a possibility of a long-running in-kernel loop without
preemption by some other task, cond_resched_rcu_qs() is required.

I welcome your deeper investigation -- I am very much treating symptoms
here, which might or might not have any relationship to fixing underlying
problems.

							Thanx, Paul