Re: [patch v3 -mm 1/3] memcg: integrate soft reclaim tighter with zone shrinking code

[Glauber Costa <glommer@parallels.com>]

On 05/13/2013 11:46 AM, Michal Hocko wrote:
> Memcg soft reclaim has been traditionally triggered from the global
> reclaim paths before calling shrink_zone. mem_cgroup_soft_limit_reclaim
> then picked up a group which exceeds the soft limit the most and
> reclaimed it with 0 priority to reclaim at least SWAP_CLUSTER_MAX pages.
> 
> The infrastructure requires per-node-zone trees which hold over-limit
> groups and keep them up-to-date (via memcg_check_events) which is not
> cost free. Although this overhead hasn't turned out to be a bottle neck
> the implementation is suboptimal because mem_cgroup_update_tree has no
> idea which zones consumed memory over the limit so we could easily end
> up having a group on a node-zone tree having only few pages from that
> node-zone.
> 
> This patch doesn't try to fix node-zone trees management because it
> seems that integrating soft reclaim into zone shrinking sounds much
> easier and more appropriate for several reasons.
> First of all 0 priority reclaim was a crude hack which might lead to
> big stalls if the group's LRUs are big and hard to reclaim (e.g. a lot
> of dirty/writeback pages).
> Soft reclaim should be applicable also to the targeted reclaim which is
> awkward right now without additional hacks.
> Last but not least the whole infrastructure eats quite some code.
> 
> After this patch shrink_zone is done in 2 passes. First it tries to do the
> soft reclaim if appropriate (only for global reclaim for now to keep
> compatible with the original state) and fall back to ignoring soft limit
> if no group is eligible to soft reclaim or nothing has been scanned
> during the first pass. Only groups which are over their soft limit or
> any of their parents up the hierarchy is over the limit are considered
> eligible during the first pass.
> 
> Soft limit tree which is not necessary anymore will be removed in the
> follow up patch to make this patch smaller and easier to review.
> 
> Changes since v1
> - __shrink_zone doesn't return the number of shrunk groups as nr_scanned
>   test covers both no groups scanned and no pages from the required zone
>   as pointed by Johannes
> 
> Signed-off-by: Michal Hocko <mhocko@suse.cz>

Patch looks fine to me

Reviewed-by: Glauber Costa <glommer@openvz.org>