Re: [PATCH 1/4] memcg, mm: introduce lowlimit reclaim

Johannes Weiner
2014-05-02T15:04:50Z

On Fri, May 02, 2014 at 04:15:15PM +0200, Michal Hocko wrote:
> On Fri 02-05-14 09:01:18, Johannes Weiner wrote:
> > On Fri, May 02, 2014 at 02:07:15PM +0200, Michal Hocko wrote:
> > > On Fri 02-05-14 11:36:28, Michal Hocko wrote:
> > > > On Wed 30-04-14 18:55:50, Johannes Weiner wrote:
> > > > > On Mon, Apr 28, 2014 at 02:26:42PM +0200, Michal Hocko wrote:
> > > > > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > > > > > index 19d620b3d69c..40e517630138 100644
> > > > > > --- a/mm/memcontrol.c
> > > > > > +++ b/mm/memcontrol.c
> > > > > > @@ -2808,6 +2808,29 @@ static struct mem_cgroup *mem_cgroup_lookup(unsigned short id)
> > > > > > return mem_cgroup_from_id(id);
> > > > > > }
> > > > > >
> > > > > > +/**
> > > > > > + * mem_cgroup_reclaim_eligible - checks whether given memcg is eligible for the
> > > > > > + * reclaim
> > > > > > + * @memcg: target memcg for the reclaim
> > > > > > + * @root: root of the reclaim hierarchy (null for the global reclaim)
> > > > > > + *
> > > > > > + * The given group is reclaimable if it is above its low limit and the same
> > > > > > + * applies for all parents up the hierarchy until root (including).
> > > > > > + */
> > > > > > +bool mem_cgroup_reclaim_eligible(struct mem_cgroup *memcg,
> > > > > > + struct mem_cgroup *root)
> > > > >
> > > > > Could you please rename this to something that is more descriptive in
> > > > > the reclaim callsite? How about mem_cgroup_within_low_limit()?
> > > >
> > > > I have intentionally used somethig that is not low_limit specific. The
> > > > generic reclaim code does't have to care about the reason why a memcg is
> > > > not reclaimable. I agree that having follow_low_limit paramter explicit
> > > > and mem_cgroup_reclaim_eligible not is messy. So something should be
> > > > renamed. I would probably go with s@follow_low_limit@check_reclaim_eligible@
> > > > but I do not have a strong preference.
> > >
> > > What about this?
> >
> > I really don't like it.
> >
> > Yes, we should be hiding implementation details, but we should stop
> > treating memcg like an alien in this code. The VM code obviously
> > doesn't have to know HOW the guarantees are exactly implemented, but
> > it's a perfectly fine *concept* that can be known outside of memcg:
> >
> > shrink_zone:
> > for each memcg in system:
> > if mem_cgroup_within_guarantee(memcg):
> > continue
> > reclaim(memcg-zone)
> >
> > is perfectly understandable and makes it easier to reason about the
> > behavior of the reclaim code. If I just see !mem_cgroup_eligible(), I
> > don't know if this affects the scenario I'm thinking about at all.
> >
> > It's obscuring useful information for absolutely no benefit. If you
> > burden the reclaim code with a callback, you better explain what you
> > are doing. You owe it to the reader.
>
> OK fair enough, what about the following?

Thanks, that's much better IMO.

> @@ -2215,8 +2215,18 @@ static inline bool should_continue_reclaim(struct zone *zone,
> }
> }
>
> +/**
> + * __shrink_zone - shrinks a given zone
> + *
> + * @zone: zone to shrink
> + * @sc: scan control with additional reclaim parameters
> + * @force_memcg_guarantee: do not reclaim memcgs which are within their memory
> + * guarantee
> + *
> + * Returns the number of reclaimed memcgs.
> + */
> static unsigned __shrink_zone(struct zone *zone, struct scan_control *sc,
> - bool follow_low_limit)
> + bool force_memcg_guarantee)
> {
> unsigned long nr_reclaimed, nr_scanned;
> unsigned nr_scanned_groups = 0;
> @@ -2236,12 +2246,9 @@ static unsigned __shrink_zone(struct zone *zone, struct scan_control *sc,
> do {
> struct lruvec *lruvec;
>
> - /*
> - * Memcg might be under its low limit so we have to
> - * skip it during the first reclaim round
> - */
> - if (follow_low_limit &&
> - !mem_cgroup_reclaim_eligible(memcg, root)) {
> + /* Memcg might be protected from the reclaim */
> + if (force_memcg_guarantee &&

respect_? consider_?

force sounds like something the second round would do -- force reclaim
despite guarantees... But then again, I'm still for removing that 2nd
force cycle, so I don't care too strongly about that name (yet) :-)