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    SubjectRe: [v5 4/4] mm, oom, docs: describe the cgroup-aware OOM killer
    On Tue, 15 Aug 2017, Roman Gushchin wrote:

    > > > diff --git a/Documentation/cgroup-v2.txt b/Documentation/cgroup-v2.txt
    > > > index dec5afdaa36d..22108f31e09d 100644
    > > > --- a/Documentation/cgroup-v2.txt
    > > > +++ b/Documentation/cgroup-v2.txt
    > > > @@ -48,6 +48,7 @@ v1 is available under Documentation/cgroup-v1/.
    > > > 5-2-1. Memory Interface Files
    > > > 5-2-2. Usage Guidelines
    > > > 5-2-3. Memory Ownership
    > > > + 5-2-4. Cgroup-aware OOM Killer
    > >
    > > Random curiousness, why cgroup-aware oom killer and not memcg-aware oom
    > > killer?
    >
    > I don't think we use the term "memcg" somewhere in v2 docs.
    > Do you think that "Memory cgroup-aware OOM killer" is better?
    >

    I think it would be better to not describe it as its own entity, but
    rather a part of how the memory cgroup works, so simply describing it in
    section 5-2, perhaps as its own subsection, as how the oom killer works
    when using the memory cgroup is sufficient. I wouldn't separate it out as
    a distinct cgroup feature in the documentation.

    > > > + cgroups. The default is "0".
    > > > +
    > > > + Defines whether the OOM killer should treat the cgroup
    > > > + as a single entity during the victim selection.
    > >
    > > Isn't this true independent of the memory.oom_kill_all_tasks setting?
    > > The cgroup aware oom killer will consider memcg's as logical units when
    > > deciding what to kill with or without memory.oom_kill_all_tasks, right?
    > >
    > > I think you cover this fact in the cgroup aware oom killer section below
    > > so this might result in confusion if described alongside a setting of
    > > memory.oom_kill_all_tasks.
    > >

    I assume this is fixed so that it's documented that memory cgroups are
    considered logical units by the oom killer and that
    memory.oom_kill_all_tasks is separate? The former defines the policy on
    how a memory cgroup is targeted and the latter defines the mechanism it
    uses to free memory.

    > > > + If set, OOM killer will kill all belonging tasks in
    > > > + corresponding cgroup is selected as an OOM victim.
    > >
    > > Maybe
    > >
    > > "If set, the OOM killer will kill all threads attached to the memcg if
    > > selected as an OOM victim."
    > >
    > > is better?
    >
    > Fixed to the following (to conform with core v2 concepts):
    > If set, OOM killer will kill all processes attached to the cgroup
    > if selected as an OOM victim.
    >

    Thanks.

    > > > +Cgroup-aware OOM Killer
    > > > +~~~~~~~~~~~~~~~~~~~~~~~
    > > > +
    > > > +Cgroup v2 memory controller implements a cgroup-aware OOM killer.
    > > > +It means that it treats memory cgroups as first class OOM entities.
    > > > +
    > > > +Under OOM conditions the memory controller tries to make the best
    > > > +choise of a victim, hierarchically looking for the largest memory
    > > > +consumer. By default, it will look for the biggest task in the
    > > > +biggest leaf cgroup.
    > > > +
    > > > +Be default, all cgroups have oom_priority 0, and OOM killer will
    > > > +chose the largest cgroup recursively on each level. For non-root
    > > > +cgroups it's possible to change the oom_priority, and it will cause
    > > > +the OOM killer to look athe the priority value first, and compare
    > > > +sizes only of cgroups with equal priority.
    > >
    > > Maybe some description of "largest" would be helpful here? I think you
    > > could briefly describe what is accounted for in the decisionmaking.
    >
    > I'm afraid that it's too implementation-defined to be described.
    > Do you have an idea, how to describe it without going too much into details?
    >

    The point is that "largest cgroup" is ambiguous here: largest in what
    sense? The cgroup with the largest number of processes attached? Using
    the largest amount of memory?

    I think the documentation should clearly define that the oom killer
    selects the memory cgroup that has the most memory managed at each level.

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