Re: [PATCH v2 1/2] mm, oom: refactor dump_tasks for memcg OOMs

[Michal Hocko <mhocko@kernel.org>]

On Mon 17-06-19 16:12:06, Shakeel Butt wrote:
> dump_tasks() currently goes through all the processes present on the
> system even for memcg OOMs. Change dump_tasks() similar to
> select_bad_process() and use mem_cgroup_scan_tasks() to selectively
> traverse the processes of the memcgs during memcg OOM.

The changelog is quite modest to be honest. I would go with

"
dump_tasks() traverses all the existing processes even for the memcg OOM
context which is not only unnecessary but also wasteful. This imposes
a long RCU critical section even from a contained context which can be
quite disruptive.

Change dump_tasks() to be aligned with select_bad_process and use
mem_cgroup_scan_tasks to selectively traverse only processes of the
target memcg hierarchy during memcg OOM.
"

> Signed-off-by: Shakeel Butt <shakeelb@google.com>

Acked-by: Michal Hocko <mhocko@suse.com>

Thanks!

> ---
> Changelog since v1:
> - Divide the patch into two patches.
> 
>  mm/oom_kill.c | 68 ++++++++++++++++++++++++++++++---------------------
>  1 file changed, 40 insertions(+), 28 deletions(-)
> 
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index 05aaa1a5920b..bd80997e0969 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -385,10 +385,38 @@ static void select_bad_process(struct oom_control *oc)
>  	oc->chosen_points = oc->chosen_points * 1000 / oc->totalpages;
>  }
>  
> +static int dump_task(struct task_struct *p, void *arg)
> +{
> +	struct oom_control *oc = arg;
> +	struct task_struct *task;
> +
> +	if (oom_unkillable_task(p, NULL, oc->nodemask))
> +		return 0;
> +
> +	task = find_lock_task_mm(p);
> +	if (!task) {
> +		/*
> +		 * This is a kthread or all of p's threads have already
> +		 * detached their mm's.  There's no need to report
> +		 * them; they can't be oom killed anyway.
> +		 */
> +		return 0;
> +	}
> +
> +	pr_info("[%7d] %5d %5d %8lu %8lu %8ld %8lu         %5hd %s\n",
> +		task->pid, from_kuid(&init_user_ns, task_uid(task)),
> +		task->tgid, task->mm->total_vm, get_mm_rss(task->mm),
> +		mm_pgtables_bytes(task->mm),
> +		get_mm_counter(task->mm, MM_SWAPENTS),
> +		task->signal->oom_score_adj, task->comm);
> +	task_unlock(task);
> +
> +	return 0;
> +}
> +
>  /**
>   * dump_tasks - dump current memory state of all system tasks
> - * @memcg: current's memory controller, if constrained
> - * @nodemask: nodemask passed to page allocator for mempolicy ooms
> + * @oc: pointer to struct oom_control
>   *
>   * Dumps the current memory state of all eligible tasks.  Tasks not in the same
>   * memcg, not in the same cpuset, or bound to a disjoint set of mempolicy nodes
> @@ -396,37 +424,21 @@ static void select_bad_process(struct oom_control *oc)
>   * State information includes task's pid, uid, tgid, vm size, rss,
>   * pgtables_bytes, swapents, oom_score_adj value, and name.
>   */
> -static void dump_tasks(struct mem_cgroup *memcg, const nodemask_t *nodemask)
> +static void dump_tasks(struct oom_control *oc)
>  {
> -	struct task_struct *p;
> -	struct task_struct *task;
> -
>  	pr_info("Tasks state (memory values in pages):\n");
>  	pr_info("[  pid  ]   uid  tgid total_vm      rss pgtables_bytes swapents oom_score_adj name\n");
> -	rcu_read_lock();
> -	for_each_process(p) {
> -		if (oom_unkillable_task(p, memcg, nodemask))
> -			continue;
>  
> -		task = find_lock_task_mm(p);
> -		if (!task) {
> -			/*
> -			 * This is a kthread or all of p's threads have already
> -			 * detached their mm's.  There's no need to report
> -			 * them; they can't be oom killed anyway.
> -			 */
> -			continue;
> -		}
> +	if (is_memcg_oom(oc))
> +		mem_cgroup_scan_tasks(oc->memcg, dump_task, oc);
> +	else {
> +		struct task_struct *p;
>  
> -		pr_info("[%7d] %5d %5d %8lu %8lu %8ld %8lu         %5hd %s\n",
> -			task->pid, from_kuid(&init_user_ns, task_uid(task)),
> -			task->tgid, task->mm->total_vm, get_mm_rss(task->mm),
> -			mm_pgtables_bytes(task->mm),
> -			get_mm_counter(task->mm, MM_SWAPENTS),
> -			task->signal->oom_score_adj, task->comm);
> -		task_unlock(task);
> +		rcu_read_lock();
> +		for_each_process(p)
> +			dump_task(p, oc);
> +		rcu_read_unlock();
>  	}
> -	rcu_read_unlock();
>  }
>  
>  static void dump_oom_summary(struct oom_control *oc, struct task_struct *victim)
> @@ -458,7 +470,7 @@ static void dump_header(struct oom_control *oc, struct task_struct *p)
>  			dump_unreclaimable_slab();
>  	}
>  	if (sysctl_oom_dump_tasks)
> -		dump_tasks(oc->memcg, oc->nodemask);
> +		dump_tasks(oc);
>  	if (p)
>  		dump_oom_summary(oc, p);
>  }
> -- 
> 2.22.0.410.gd8fdbe21b5-goog
> 

-- 
Michal Hocko
SUSE Labs