Re: [PATCH] mm, vmscan: add cond_resched into shrink_node_memcg

[Michal Hocko]

[CC Paul - sorry I've tried to save you from more emails...]

On Mon 05-12-16 23:44:27, Balbir Singh wrote:
> >
> > Hi,
> > there were multiple reportes of the similar RCU stalls. Only Boris has
> > confirmed that this patch helps in his workload. Others might see a
> > slightly different issue and that should be investigated if it is the
> > case. As pointed out by Paul [1] cond_resched might be not sufficient
> > to silence RCU stalls because that would require a real scheduling.
> > This is a separate problem, though, and Paul is working with Peter [2]
> > to resolve it.
> >
> > Anyway, I believe that this patch should be a good start because it
> > really seems that nr_taken=0 during the LRU isolation can be triggered
> > in the real life. All reporters are agreeing to start seeing this issue
> > when moving on to 4.8 kernel which might be just a coincidence or a
> > different behavior of some subsystem. Well, MM has moved from zone to
> > node reclaim but I couldn't have found any direct relation to that
> > change.
> >
> > [1] http://lkml.kernel.org/r/20161130142955.GS3924@xxxxxxxxxxxxxxxxxx
> > [2] http://lkml.kernel.org/r/20161201124024.GB3924@xxxxxxxxxxxxxxxxxx
> >
> >  mm/vmscan.c | 2 ++
> >  1 file changed, 2 insertions(+)
> >
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index c05f00042430..c4abf08861d2 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -2362,6 +2362,8 @@ static void shrink_node_memcg(struct pglist_data *pgdat, struct mem_cgroup *memc
> >                         }
> >                 }
> >
> > +               cond_resched();
> > +
> 
> I see a cond_resched_rcu_qs() as a part of linux next inside the while
> (nr[..]) loop.

This is a left over from Paul's initial attempt to fix this issue. I
expect him to drop his patch from his tree. He has considered it
experimental anyway.

> Do we need this as well?

Paul is working with Peter to make cond_resched general and cover RCU
stalls even when cond_resched doesn't schedule because there is no
runnable task.

-- 
Michal Hocko
SUSE Labs