Re: [patch 2/8] mm: memcg-aware global reclaim

Johannes Weiner
2011-06-08T09:31:11Z

On Tue, Jun 07, 2011 at 08:25:19AM -0400, Christoph Hellwig wrote:
> A few small nitpicks:
>
> > +struct mem_cgroup *mem_cgroup_hierarchy_walk(struct mem_cgroup *root,
> > + struct mem_cgroup *prev)
> > +{
> > + struct mem_cgroup *mem;
> > +
> > + if (mem_cgroup_disabled())
> > + return NULL;
> > +
> > + if (!root)
> > + root = root_mem_cgroup;
> > + /*
> > + * Even without hierarchy explicitely enabled in the root
> > + * memcg, it is the ultimate parent of all memcgs.
> > + */
> > + if (!(root == root_mem_cgroup || root->use_hierarchy))
> > + return root;
>
> The logic here reads a bit weird, why not simply:
>
> /*
> * Even without hierarchy explicitely enabled in the root
> * memcg, it is the ultimate parent of all memcgs.
> */
> if (!root || root == root_mem_cgroup)
> return root_mem_cgroup;
> if (root->use_hierarchy)
> return root;

What you are proposing is not equivalent, so... case in point! It's
meant to do the hierarchy walk for when foo->use_hierarchy, obviously,
but ALSO for root_mem_cgroup, which is parent to everyone else even
without use_hierarchy set. I changed it to read like this:

if (!root)
root = root_mem_cgroup;
if (!root->use_hierarchy && root != root_mem_cgroup)
return root;
/* actually iterate hierarchy */

Does that make more sense?

Another alternative would be

if (root->use_hierarchy || root == root_mem_cgroup) {
/* most of the function body */
}

but that quickly ends up with ugly linewraps...

> > /*
> > * This is a basic per-zone page freer. Used by both kswapd and direct reclaim.
> > */
> > -static void shrink_zone(int priority, struct zone *zone,
> > - struct scan_control *sc)
> > +static void do_shrink_zone(int priority, struct zone *zone,
> > + struct scan_control *sc)
>
> It actually is the per-memcg shrinker now, and thus should be called
> shrink_memcg.

Per-zone per-memcg, actually. shrink_zone_memcg?

> > + sc->mem_cgroup = mem;
> > + do_shrink_zone(priority, zone, sc);
>
> Any passing the mem_cgroup explicitly instead of hiding it in the
> scan_control would make that much more obvious. If there's a good
> reason to pass it in the structure the same probably applies to the
> zone and priority, too.

Stack frame size, I guess. But unreadable code can't be the answer to
this problem. I'll try to pass it explicitely and see what the damage
is.

> Shouldn't we also have a non-cgroups stub of shrink_zone to directly
> call do_shrink_zone/shrink_memcg with a NULL memcg and thus optimize
> the whole loop away for it?

On !CONFIG_MEMCG, the code in shrink_zone() looks effectively like
this:

first = mem = NULL;
for (;;) {
sc->mem_cgroup = mem;
do_shrink_zone()
if (reclaimed enough)
break;
mem = NULL;
if (first == mem)
break;
}

I have gcc version 4.6.0 20110530 (Red Hat 4.6.0-9) (GCC) on this
machine, and it manages to optimize the loop away completely.

The only increase in code size I could see was from all callers having
to do the extra sc->mem_cgroup = NULL. But I guess there is no way
around this.