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

[Michal Hocko <mhocko@suse.cz>]

On Mon 10-06-13 12:54:26, Tejun Heo wrote:
> Hello, Michal.
> 
> On Mon, Jun 10, 2013 at 10:02:08AM +0200, Michal Hocko wrote:
> > Sure a next visit on the same root subtree (same node, zone and prio)
> > would css_put it but what if that root goes away itself. Still fixable,
> > if every group checks its own cached iters and css_put everybody but
> > that is even uglier. So doing the up-the-hierarchy cleanup in RCU
> > callback is much easier.
> 
> Ooh, right, we don't need cleanup of the cached cursors on destruction
> if we get this correct - especially if we make cursors point to the
> next cgroup to visit as self is always the first one to visit. 

You would need to pin the next-to-visit memcg as well, so you need a
cleanup on the removal.

> Yeah, if we can do away with that, doing that way is definitely
> better.

The only advantage I can see from next-to-visit caching is that the
destruction path can reuse __mem_cgroup_iter_next unlike last_visited
which would need to develop a code to get the previous member. Maybe it
is worth a try.
-- 
Michal Hocko
SUSE Labs

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