[Devel] [PATCH rh7 v4 0/9] mm/mem_cgroup_iter: Reduce the number of iterator restarts upon cgroup removals

[Konstantin Khorenko]

May thanks to Kirill Tkhai for his bright ideas and review!

Problem description from the user point of view:
  * the Node is slow
  * the Node has a lot of free RAM
  * the Node has a lot of swapin/swapout
  * kswapd is always running

Problem in a nutshell from technical point of view:
  * kswapd is looping in shrink_zone() inside the loop
      do {} while ((memcg = mem_cgroup_iter(root, memcg, &reclaim)));
    (and never goes trough the outer loop)
  * there are a quite a number of memory cgroups of the Node (~1000)
  * some cgroups are hard to reclaim (reclaim may take ~3 seconds),
    this is because of very busy disk due to permanent swapin/swapout
  * mem_cgroup_iter() does not have success scanning all cgroups
    in a row, it restarts from the root cgroup one time after
    another (after different number of cgroups scanned)

Q: Why does mem_cgroup_iter() restart from the root memcg?
A: Because it is invalidated once some memory cgroup is
   destroyed on the Node.
   Note: ANY memory cgroup destroy on the Node leads to iter
   restart.

The following patchset solves this problem in the following way:
there is no need to restart the iter until we see the iter has
the position which is exactly the memory cgroup being destroyed.

The patchset ensures the iter->last_visited is NULL-ified on
invalidation and thus restarts only in the unlikely case when
the iter points to the memcg being destroyed.

https://jira.sw.ru/browse/PSBM-123655

v2 changes:
 - reverted 2 patches in this code which were focused on syncronizing
   updates of iter->last_visited and ->last_dead_count
   (as we are getting rid of iter->last_dead_count at all)
 - use rcu primitives to access iter->last_visited

v3 changes:
 - more comments explaining the locking scheme
 - use rcu_read_{lock,unlock}_sched() in mem_cgroup_iter()
   for syncronization with iterator invalidation func
 - do not use rcu_read_{lock/unlock}() wrap in iterator invalidation func
   as it protects nothing

v4 changes:
 - extended comment why iter invalidation function must see all
   pointers to dying memcg and no pointer to it can be written later

Konstantin Khorenko (9):
  Revert "mm/memcg: fix css_tryget(),css_put() imbalance"
  Revert "mm/memcg: use seqlock to protect reclaim_iter updates"
  mm/mem_cgroup_iter: Make 'iter->last_visited' a bit more stable
  mm/mem_cgroup_iter: Always assign iter->last_visited under rcu
  mm/mem_cgroup_iter: Provide _iter_invalidate() the dying memcg as an
    argument
  mm/mem_cgroup_iter: NULL-ify 'last_visited' for invalidated iterators
  mm/mem_cgroup_iter: Don't bother checking 'dead_count' anymore
  mm/mem_cgroup_iter: Cleanup mem_cgroup_iter_load()
  mm/mem_cgroup_iter: Drop dead_count related infrastructure

 mm/memcontrol.c | 208 ++++++++++++++++++++++++++++++++++--------------
 1 file changed, 150 insertions(+), 58 deletions(-)

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2.24.3