Re: [PATCH 3/3] memcg: simplify mem_cgroup_reclaim_iter

[Michal Hocko <mhocko@suse.cz>]

On Tue 11-06-13 00:44:04, Tejun Heo wrote:
> On Tue, Jun 11, 2013 at 09:27:43AM +0200, Michal Hocko wrote:
> >           .
> >           .
> >           .
> >           A (cached=E)
> >          /|\____________
> >         / |             \
> > 	 B  D (cached=E)   F<
> > 	/   |               \
> >      C<   E                G
> >           ^
> > 	 removed
> > 
> > * D level cache - nobody left for either approach approach
> > * A level is 
> > 	- F for next-to-visit
> > 	- C for last_visited
> > 
> > You have to get up the hierarchy and handle root cgroup as a special
> > case for !root->use_hierarchy. Once you have non-NULL new cache the it
> > can be propagated without a new search (which I haven't realized when
> > working on this approach the last time - not that it would safe some
> > code in the end).
> > 
> > Makes sense?
> 
> I don't think we're talking about the same thing.  I wasn't talking
> about skipping walking up the hierarchy (differently depending on
> use_hierarchy of course) when E is removed.  I was talking about
> skipped cleaning E's cache when removing E as it's guaranteed to be
> empty by then.

Ahh, sorry I have misread your reply then. You are right that caching
the next-to-visit would keep its own cache clean.

> The difference between caching the last and next one is that if we put
> the last one in the cache, E's cache could be pointing to itself and
> needs to be scanned.

Right.

> Not a big difference either way but if you combine that with the need
> for special rewinding which will basically come down to traversing the
> sibling list again, pointing to the next entry is just easier.
> 
> Anyways, I think we're getting too deep into details but one more
> thing, what do you mean by "non-NULL new cache"?

If you replace cached memcg by a new (non-NULL) one then all the parents
up the hierarchy can reuse the same replacement and do not have to
search again.

-- 
Michal Hocko
SUSE Labs

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