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

Christoph Hellwig
2011-06-09T09:26:32Z

On Wed, Jun 08, 2011 at 11:30:46AM +0200, Johannes Weiner wrote:
> 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?

It does, sorry for misparsing it. The thing that I really hated was
the conditional assignment of root. Can we clean this up somehow
by making the caller pass root_mem_cgroup in the case where it
passes root right now, or at least always pass NULL when it means
root_mem_cgroup.

Note really that important in the end, it just irked me when I looked
over it, especially the conditional assigned of root to root_mem_cgroup,
and then a little later checking for the equality of the two.

Thinking about it it's probably better left as-is for now to not
complicate the series, and maybe revisit it later once things have
settled a bit.

> > It actually is the per-memcg shrinker now, and thus should be called
> > shrink_memcg.
>
> Per-zone per-memcg, actually. shrink_zone_memcg?

Sounds fine to me.

> 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.

Ok, good enough.