Re: [RFC PATCH v1] mm: oom: introduce cpuset oom

[David Rientjes]

On Wed, 21 Sep 2022, Gang Li wrote:

> cpuset confine processes to processor and memory node subsets.
> When a process in cpuset triggers oom, it may kill a completely
> irrelevant process on another numa node, which will not release any
> memory for this cpuset.
> 
> It seems that `CONSTRAINT_CPUSET` is not really doing much these
> days. Using CONSTRAINT_CPUSET, we can easily achieve node aware oom
> killing by selecting victim from the cpuset which triggers oom.
> 
> Suggested-by: Michal Hocko <mhocko@xxxxxxxx>
> Signed-off-by: Gang Li <ligang.bdlg@xxxxxxxxxxxxx>

Hmm, is this the right approach?

If a cpuset results in a oom condition, is there a reason why we'd need to 
find a process from within that cpuset to kill?  I think the idea is to 
free memory on the oom set of nodes (cpuset.mems) and that can happen by 
killing a process that is not a member of this cpuset.

I understand the challenges of creating a NUMA aware oom killer to target 
memory that is actually resident on an oom node, but this approach doesn't 
seem right and could actually lead to pathological cases where a small 
process trying to fork in an otherwise empty cpuset is repeatedly oom 
killing when we'd actually prefer to kill a single large process.

> ---
> This idea comes from a previous patch:
> mm, oom: Introduce per numa node oom for CONSTRAINT_MEMORY_POLICY
> https://lore.kernel.org/all/YoJ%2FioXwGTdCywUE@xxxxxxxxxxxxxx/
> 
> Any comments are welcome.
> ---
>  include/linux/cpuset.h |  6 ++++++
>  kernel/cgroup/cpuset.c | 17 +++++++++++++++++
>  mm/oom_kill.c          |  4 ++++
>  3 files changed, 27 insertions(+)
> 
> diff --git a/include/linux/cpuset.h b/include/linux/cpuset.h
> index d58e0476ee8e..7475f613ab90 100644
> --- a/include/linux/cpuset.h
> +++ b/include/linux/cpuset.h
> @@ -178,6 +178,8 @@ static inline void set_mems_allowed(nodemask_t nodemask)
>  	task_unlock(current);
>  }
>  
> +int cpuset_cgroup_scan_tasks(int (*fn)(struct task_struct *, void *), void *arg);
> +
>  #else /* !CONFIG_CPUSETS */
>  
>  static inline bool cpusets_enabled(void) { return false; }
> @@ -299,6 +301,10 @@ static inline bool read_mems_allowed_retry(unsigned int seq)
>  	return false;
>  }
>  
> +static inline int cpuset_cgroup_scan_tasks(int (*fn)(struct task_struct *, void *), void *arg)
> +{
> +	return 0;
> +}
>  #endif /* !CONFIG_CPUSETS */
>  
>  #endif /* _LINUX_CPUSET_H */
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index b474289c15b8..1f1238b4276d 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -3943,6 +3943,23 @@ void cpuset_print_current_mems_allowed(void)
>  	rcu_read_unlock();
>  }
>  
> +int cpuset_cgroup_scan_tasks(int (*fn)(struct task_struct *, void *), void *arg)
> +{
> +	int ret = 0;
> +	struct cgroup *cgrp;
> +	struct css_task_iter it;
> +	struct task_struct *task;
> +
> +	rcu_read_lock();
> +	css_task_iter_start(&(task_cs(current)->css), CSS_TASK_ITER_PROCS, &it);
> +	while (!ret && (task = css_task_iter_next(&it)))
> +		ret = fn(task, arg);
> +	css_task_iter_end(&it);
> +	rcu_read_unlock();
> +
> +	return ret;
> +}
> +
>  /*
>   * Collection of memory_pressure is suppressed unless
>   * this flag is enabled by writing "1" to the special
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index 46e7e073f137..8cea787b359c 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -367,6 +367,8 @@ static void select_bad_process(struct oom_control *oc)
>  
>  	if (is_memcg_oom(oc))
>  		mem_cgroup_scan_tasks(oc->memcg, oom_evaluate_task, oc);
> +	else if (oc->constraint == CONSTRAINT_CPUSET)
> +		cpuset_cgroup_scan_tasks(oom_evaluate_task, oc);
>  	else {
>  		struct task_struct *p;
>  
> @@ -427,6 +429,8 @@ static void dump_tasks(struct oom_control *oc)
>  
>  	if (is_memcg_oom(oc))
>  		mem_cgroup_scan_tasks(oc->memcg, dump_task, oc);
> +	else if (oc->constraint == CONSTRAINT_CPUSET)
> +		cpuset_cgroup_scan_tasks(dump_task, oc);
>  	else {
>  		struct task_struct *p;
>  
> -- 
> 2.20.1
> 
> 
>