linux-kernel - Re: [PATCH v2] mm/oom_kill: show oom eligibility when displaying the current memory state of all tasks

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Message-ID: <6fc8beef-4dbb-b49a-4653-90fe564941a6@google.com>
Date:   Sun, 13 Jun 2021 16:47:49 -0700 (PDT)
From:   David Rientjes <rientjes@...gle.com>
To:     Aaron Tomlin <atomlin@...hat.com>
cc:     linux-mm@...ck.org, akpm@...ux-foundation.org, vbabka@...e.cz,
        mhocko@...e.com, penguin-kernel@...ove.sakura.ne.jp,
        llong@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mm/oom_kill: show oom eligibility when displaying
 the current memory state of all tasks

On Sat, 12 Jun 2021, Aaron Tomlin wrote:

> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index eefd3f5fde46..094b7b61d66f 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -160,6 +160,27 @@ static inline bool is_sysrq_oom(struct oom_control *oc)
>  	return oc->order == -1;
>  }
>  
> +/**
> + * is_task_eligible_oom - determine if and why a task cannot be OOM killed
> + * @tsk: task to check
> + *
> + * Needs to be called with task_lock().
> + */
> +static const char * is_task_oom_eligible(struct task_struct *p)

You should be able to just return a char.

> +{
> +	long adj;
> +
> +	adj = (long)p->signal->oom_score_adj;
> +	if (adj == OOM_SCORE_ADJ_MIN)
> +		return "S";

The value is already printed in the task dump, this doesn't look to add 
any information.

> +	else if (test_bit(MMF_OOM_SKIP, &p->mm->flags)
> +		return "R";

We should be doing the task dump only when we're killing a victim (unless 
we're panicking), so something else has been chosen.  Since we would have 
oom killed a process with MMF_OOM_SKIP already, can we simply choose to 
not print a line for this process?

> +	else if (in_vfork(p))
> +		return "V";

Is this a transition state that we can simply disregard from the output as 
well unless/until it becomes eligible?

> +	else
> +		return "";
> +}
> +
>  /* return true if the task is not adequate as candidate victim task. */
>  static bool oom_unkillable_task(struct task_struct *p)
>  {
> @@ -401,12 +422,13 @@ static int dump_task(struct task_struct *p, void *arg)
>  		return 0;
>  	}
>  
> -	pr_info("[%7d] %5d %5d %8lu %8lu %8ld %8lu         %5hd %s\n",
> +	pr_info("[%7d] %5d %5d %8lu %8lu %8ld %8lu         %5hd %13s %s\n",

13 characters for one char output?

>  		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->signal->oom_score_adj, is_task_oom_eligible(task),
> +		task->comm);
>  	task_unlock(task);
>  
>  	return 0;
> @@ -420,12 +442,13 @@ static int dump_task(struct task_struct *p, void *arg)
>   * memcg, not in the same cpuset, or bound to a disjoint set of mempolicy nodes
>   * are not shown.
>   * State information includes task's pid, uid, tgid, vm size, rss,
> - * pgtables_bytes, swapents, oom_score_adj value, and name.
> + * pgtables_bytes, swapents, oom_score_adj value, oom eligibility status
> + * and name.
>   */
>  static void dump_tasks(struct oom_control *oc)
>  {
>  	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");
> +	pr_info("[  pid  ]   uid  tgid total_vm      rss pgtables_bytes swapents oom_score_adj oom eligible? name\n");

Field names are single words.

>  
>  	if (is_memcg_oom(oc))
>  		mem_cgroup_scan_tasks(oc->memcg, dump_task, oc);