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SubjectRe: [patch v3 -mm 3/6] mm, memcg: add hierarchical usage oom policy
On Fri, Jul 13, 2018 at 04:07:29PM -0700, David Rientjes wrote:
> One of the three significant concerns brought up about the cgroup aware
> oom killer is that its decisionmaking is completely evaded by creating
> subcontainers and attaching processes such that the ancestor's usage does
> not exceed another cgroup on the system.
>
> Consider the example from the previous patch where "memory" is set in
> each mem cgroup's cgroup.controllers:
>
> mem cgroup cgroup.procs
> ========== ============
> /cg1 1 process consuming 250MB
> /cg2 3 processes consuming 100MB each
> /cg3/cg31 2 processes consuming 100MB each
> /cg3/cg32 2 processes consuming 100MB each
>
> If memory.oom_policy is "cgroup", a process from /cg2 is chosen because it
> is in the single indivisible memory consumer with the greatest usage.
>
> The true usage of /cg3 is actually 400MB, but a process from /cg2 is
> chosen because cgroups are compared individually rather than
> hierarchically.
>
> If a system is divided into two users, for example:
>
> mem cgroup memory.max
> ========== ==========
> /userA 250MB
> /userB 250MB
>
> If /userA runs all processes attached to the local mem cgroup, whereas
> /userB distributes their processes over a set of subcontainers under
> /userB, /userA will be unfairly penalized.
>
> There is incentive with cgroup v2 to distribute processes over a set of
> subcontainers if those processes shall be constrained by other cgroup
> controllers; this is a direct result of mandating a single, unified
> hierarchy for cgroups. A user may also reasonably do this for mem cgroup
> control or statistics. And, a user may do this to evade the cgroup-aware
> oom killer selection logic.
>
> This patch adds an oom policy, "tree", that accounts for hierarchical
> usage when comparing cgroups and the cgroup aware oom killer is enabled by
> an ancestor. This allows administrators, for example, to require users in
> their own top-level mem cgroup subtree to be accounted for with
> hierarchical usage. In other words, they can longer evade the oom killer
> by using other controllers or subcontainers.
>
> If an oom policy of "tree" is in place for a subtree, such as /cg3 above,
> the hierarchical usage is used for comparisons with other cgroups if
> either "cgroup" or "tree" is the oom policy of the oom mem cgroup. Thus,
> if /cg3/memory.oom_policy is "tree", one of the processes from /cg3's
> subcontainers is chosen for oom kill.
>
> Signed-off-by: David Rientjes <rientjes@google.com>
> ---
> Documentation/admin-guide/cgroup-v2.rst | 17 ++++++++++++++---
> include/linux/memcontrol.h | 5 +++++
> mm/memcontrol.c | 18 ++++++++++++------
> 3 files changed, 31 insertions(+), 9 deletions(-)
>
> diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
> --- a/Documentation/admin-guide/cgroup-v2.rst
> +++ b/Documentation/admin-guide/cgroup-v2.rst
> @@ -1113,6 +1113,10 @@ PAGE_SIZE multiple when read back.
> memory consumers; that is, they will compare mem cgroup usage rather
> than process memory footprint. See the "OOM Killer" section below.
>
> + If "tree", the OOM killer will compare mem cgroups and its subtree
> + as a single indivisible memory consumer. This policy cannot be set
> + on the root mem cgroup. See the "OOM Killer" section below.
> +
> When an OOM condition occurs, the policy is dictated by the mem
> cgroup that is OOM (the root mem cgroup for a system-wide OOM
> condition). If a descendant mem cgroup has a policy of "none", for
> @@ -1120,6 +1124,10 @@ PAGE_SIZE multiple when read back.
> the heuristic will still compare mem cgroups as indivisible memory
> consumers.
>
> + When an OOM condition occurs in a mem cgroup with an OOM policy of
> + "cgroup" or "tree", the OOM killer will compare mem cgroups with
> + "cgroup" policy individually with "tree" policy subtrees.
> +
> memory.events
> A read-only flat-keyed file which exists on non-root cgroups.
> The following entries are defined. Unless specified
> @@ -1355,7 +1363,7 @@ out of memory, its memory.oom_policy will dictate how the OOM killer will
> select a process, or cgroup, to kill. Likewise, when the system is OOM,
> the policy is dictated by the root mem cgroup.
>
> -There are currently two available oom policies:
> +There are currently three available oom policies:
>
> - "none": default, choose the largest single memory hogging process to
> oom kill, as traditionally the OOM killer has always done.
> @@ -1364,6 +1372,9 @@ There are currently two available oom policies:
> subtree as an OOM victim and kill at least one process, depending on
> memory.oom_group, from it.
>
> + - "tree": choose the cgroup with the largest memory footprint considering
> + itself and its subtree and kill at least one process.
> +
> When selecting a cgroup as a victim, the OOM killer will kill the process
> with the largest memory footprint. A user can control this behavior by
> enabling the per-cgroup memory.oom_group option. If set, it causes the
> @@ -1382,8 +1393,8 @@ Please, note that memory charges are not migrating if tasks
> are moved between different memory cgroups. Moving tasks with
> significant memory footprint may affect OOM victim selection logic.
> If it's a case, please, consider creating a common ancestor for
> -the source and destination memory cgroups and enabling oom_group
> -on ancestor layer.
> +the source and destination memory cgroups and setting a policy of "tree"
> +and enabling oom_group on an ancestor layer.
>
>
> IO
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -77,6 +77,11 @@ enum memcg_oom_policy {
> * mem cgroup as an indivisible consumer
> */
> MEMCG_OOM_POLICY_CGROUP,
> + /*
> + * Tree cgroup usage for all descendant memcg groups, treating each mem
> + * cgroup and its subtree as an indivisible consumer
> + */
> + MEMCG_OOM_POLICY_TREE,
> };
>
> struct mem_cgroup_reclaim_cookie {
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2952,7 +2952,7 @@ static void select_victim_memcg(struct mem_cgroup *root, struct oom_control *oc)
> /*
> * The oom_score is calculated for leaf memory cgroups (including
> * the root memcg).
> - * Non-leaf oom_group cgroups accumulating score of descendant
> + * Cgroups with oom policy of "tree" accumulate the score of descendant
> * leaf memory cgroups.
> */
> rcu_read_lock();
> @@ -2961,10 +2961,11 @@ static void select_victim_memcg(struct mem_cgroup *root, struct oom_control *oc)
>
> /*
> * We don't consider non-leaf non-oom_group memory cgroups
> - * as OOM victims.
> + * without the oom policy of "tree" as OOM victims.
> */
> if (memcg_has_children(iter) && iter != root_mem_cgroup &&
> - !mem_cgroup_oom_group(iter))
> + !mem_cgroup_oom_group(iter) &&
> + iter->oom_policy != MEMCG_OOM_POLICY_TREE)
> continue;

Hello, David!

I think that there is an inconsistency in the memory.oom_policy definition.
"none" and "cgroup" policies defining how the OOM scoped to this particular
memory cgroup (or system, if set on root) is handled. And all sub-tree
settings do not matter at all, right? Also, if a memory cgroup has no
memory.max set, there is no meaning in setting memory.oom_policy.

And "tree" is different. It actually changes how the selection algorithm works,
and sub-tree settings do matter in this case.

I find it very confusing.

Thanks!

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