Memory controller soft limit patches (v5)

From: Balbir Singh <[email protected]>

New Feature: Soft limits for memory resource controller.

Changelog v5...v4
1. Several changes to the reclaim logic, please see the patch 4 (reclaim on contention). I've experimented with several possibilities for reclaim and chose to come back to this due to the excellent behaviour seen while testing the patchset.
2. Reduced the overhead of soft limits on resource counters very significantly. Reaim benchmark now shows almost no drop in performance.

Changelog v4...v3
1. Adopted suggestions from Kamezawa to do a per-zone-per-node reclaim while doing soft limit reclaim. We don't record priorities while doing soft reclaim
2. Some of the overheads associated with soft limits (like calculating excess each time) is eliminated
3. The time_after(jiffies, 0) bug has been fixed
4. Tasks are throttled if the mem cgroup they belong to is being soft reclaimed and at the same time tasks are increasing the memory footprint and causing the mem cgroup to exceed its soft limit.

Changelog v3...v2
1. Implemented several review comments from Kosaki-San and Kamezawa-San Please see individual changelogs for changes

Changelog v2...v1
1. Soft limits now support hierarchies
2. Use spinlocks instead of mutexes for synchronization of the RB tree

Here is v5 of the new soft limit implementation. Soft limits is a new feature for the memory resource controller, something similar has existed in the group scheduler in the form of shares. The CPU controllers interpretation of shares is very different though.

Soft limits are the most useful feature to have for environments where the administrator wants to overcommit the system, such that only on memory contention do the limits become active. The current soft limits implementation provides a soft_limit_in_bytes interface for the memory controller and not for memory+swap controller. The implementation maintains an RB-Tree of groups that exceed their soft limit and starts reclaiming from the group that exceeds this limit by the maximum amount.

Kamezawa-San has another patchset for soft limits, but I don't like the reclaim logic of watermark based balancing of zones for global memory cgroup limits. I also don't like the data structures, a list does not scale well. Kamezawa's
objection to this patch is the cost of sorting, which is really negligible,
since the updates happen at a fixed interval (curently four times a second).
I however do like the priority feature in Kamezawa's patchset. The feature
can be easily adopted to this incrementally.

TODOs

1. The current implementation maintains the delta from the soft limit and pushes back groups to their soft limits, a ratio of delta/soft_limit might be more useful
2. It would be nice to have more targetted reclaim (in terms of pages to recalim) interface. So that groups are pushed back, close to their soft limits.

Tests
-----

I've run two memory intensive workloads with differing soft limits and seen that they are pushed back to their soft limit on contention. Their usage was their soft limit plus additional memory that they were able to grab on the system. Soft limit can take a while before we see the expected results.

The other tests I've run are
1. Deletion of groups while soft limit is in progress in the hierarchy
2. Setting the soft limit to zero and running other groups with non-zero soft limits.
3. Setting the soft limit to zero and testing if the mem cgroup is able to use available memory

Please review, comment.

Series
------

memcg-soft-limit-documentation.patch
memcg-add-soft-limit-interface.patch
memcg-organize-over-soft-limit-groups.patch
memcg-soft-limit-reclaim-on-contention.patch

	Balbir

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