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Re: [v5 4/4] mm, oom, docs: describe the cgroup-aware OOM killer

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  Re: [v5 4/4] mm, oom, docs: describe the cgroup-aware OOM killer David Rientjes <rientjes@google.com> - 2017-08-15 01:00 +0200
    Re: [v5 4/4] mm, oom, docs: describe the cgroup-aware OOM killer David Rientjes <rientjes@google.com> - 2017-08-15 23:00 +0200
      Re: [v5 4/4] mm, oom, docs: describe the cgroup-aware OOM killer David Rientjes <rientjes@google.com> - 2017-08-21 02:50 +0200

#1711545 — Re: [v5 4/4] mm, oom, docs: describe the cgroup-aware OOM killer

On Mon, 14 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?

>       5-3. IO
>         5-3-1. IO Interface Files
>         5-3-2. Writeback
> @@ -1002,6 +1003,37 @@ PAGE_SIZE multiple when read back.
>  	high limit is used and monitored properly, this limit's
>  	utility is limited to providing the final safety net.
>  
> +  memory.oom_kill_all_tasks
> +
> +	A read-write single value file which exits on non-root

s/exits/exists/

> +	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.

> +
> +	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?

> +
> +	Be default, OOM killer respect /proc/pid/oom_score_adj value
> +	-1000, and will never kill the task, unless oom_kill_all_tasks
> +	is set.
> +
> +  memory.oom_priority
> +
> +	A read-write single value file which exits on non-root

s/exits/exists/

> +	cgroups.  The default is "0".
> +
> +	An integer number within the [-10000, 10000] range,
> +	which defines the order in which the OOM killer selects victim
> +	memory cgroups.
> +
> +	OOM killer prefers memory cgroups with larger priority if they
> +	are populated with elegible tasks.

s/elegible/eligible/

> +
> +	The oom_priority value is compared within sibling cgroups.
> +
> +	The root cgroup has the oom_priority 0, which cannot be changed.
> +
>    memory.events
>  	A read-only flat-keyed file which exists on non-root cgroups.
>  	The following entries are defined.  Unless specified
> @@ -1206,6 +1238,36 @@ POSIX_FADV_DONTNEED to relinquish the ownership of memory areas
>  belonging to the affected files to ensure correct memory ownership.
>  
>  
> +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.

s/athe/at the/

Reading through this, it makes me wonder if doing s/cgroup/memcg/ over 
most of it would be better.

> +
> +But a user can change this behavior by enabling the per-cgroup
> +oom_kill_all_tasks option. If set, it causes the OOM killer treat
> +the whole cgroup as an indivisible memory consumer. In case if it's
> +selected as on OOM victim, all belonging tasks will be killed.
> +
> +Tasks in the root cgroup are treated as independent memory consumers,
> +and are compared with other memory consumers (e.g. leaf cgroups).
> +The root cgroup doesn't support the oom_kill_all_tasks feature.
> +
> +This affects both system- and cgroup-wide OOMs. For a cgroup-wide OOM
> +the memory controller considers only cgroups belonging to the sub-tree
> +of the OOM'ing cgroup.
> +
>  IO
>  --
>  

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#1712476

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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#1716050

On Thu, 17 Aug 2017, Roman Gushchin wrote:

> Hi David!
> 
> Please, find an updated version of docs patch below.
> 

Looks much better, thanks!  I think the only pending issue is discussing 
the relationship of memory.oom_kill_all_tasks with /proc/pid/oom_score_adj 
== OOM_SCORE_ADJ_MIN.

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