Re: [patch 8/8] mm: make per-memcg lru lists exclusive

[Johannes Weiner]

On Tue, Jun 07, 2011 at 08:42:13AM -0400, Christoph Hellwig wrote:
> On Wed, Jun 01, 2011 at 08:25:19AM +0200, Johannes Weiner wrote:
> > All lru list walkers have been converted to operate on per-memcg
> > lists, the global per-zone lists are no longer required.
> > 
> > This patch makes the per-memcg lists exclusive and removes the global
> > lists from memcg-enabled kernels.
> > 
> > The per-memcg lists now string up page descriptors directly, which
> > unifies/simplifies the list isolation code of page reclaim as well as
> > it saves a full double-linked list head for each page in the system.
> > 
> > At the core of this change is the introduction of the lruvec
> > structure, an array of all lru list heads.  It exists for each zone
> > globally, and for each zone per memcg.  All lru list operations are
> > now done in generic code against lruvecs, with the memcg lru list
> > primitives only doing accounting and returning the proper lruvec for
> > the currently scanned memcg on isolation, or for the respective page
> > on putback.
> 
> Wouldn't it be simpler if we always have a stub mem_cgroup_per_zone
> structure even for non-memcg kernels, and always operate on a
> single instance per node of those for non-memcg kernels?  In effect the
> lruvec almost is something like that, just adding another layer of
> abstraction.

I assume you meant 'single instance per zone'; the lruvec is this.  It
exists per zone and per mem_cgroup_per_zone so there is no difference
between memcg kernels and non-memcg ones in generic code.  But maybe
you really meant 'node' and I just don't get it?  Care to elaborate a
bit more?

> >  static inline struct mem_cgroup *try_get_mem_cgroup_from_page(struct page *page)
> > diff --git a/include/linux/mm_inline.h b/include/linux/mm_inline.h
> > index 8f7d247..43d5d9f 100644
> > --- a/include/linux/mm_inline.h
> > +++ b/include/linux/mm_inline.h
> > @@ -25,23 +25,27 @@ static inline void
> >  __add_page_to_lru_list(struct zone *zone, struct page *page, enum lru_list l,
> >  		       struct list_head *head)
> >  {
> > +	/* NOTE: Caller must ensure @head is on the right lruvec! */
> > +	mem_cgroup_lru_add_list(zone, page, l);
> >  	list_add(&page->lru, head);
> >
> >  	__mod_zone_page_state(zone, NR_LRU_BASE + l, hpage_nr_pages(page));
> > -	mem_cgroup_add_lru_list(page, l);
> >  }
> 
> This already has been a borderline-useful function before, but with the
> new changes it's not a useful helper.  Either add the code surrounding
> it includeing the PageLRU check and the normal add_page_to_lru_list
> into a new page_update_lru_pos or similar helper, or just opencode these
> bits in the only caller with a comment documenting why we are doing it.
> 
> I would tend towards the opencoding variant.

It's only one user, I'll opencode it.  That also makes for a nice
opportunity to document at the current callsite why the lruvec is
guaranteed to be the right one.
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