linux-kernel - Re: [PATCH v3] mm/oom_kill: show oom eligibility when displaying the current memory state of all tasks

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Date:   Sun, 1 Aug 2021 20:49:02 -0700 (PDT)
From:   David Rientjes <rientjes@...gle.com>
To:     Aaron Tomlin <atomlin@...hat.com>
cc:     linux-mm@...ck.org, akpm@...ux-foundation.org, mhocko@...e.com,
        penguin-kernel@...ove.sakura.ne.jp, llong@...hat.com,
        neelx@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] mm/oom_kill: show oom eligibility when displaying
 the current memory state of all tasks

On Fri, 30 Jul 2021, Aaron Tomlin wrote:

>  Documentation/admin-guide/sysctl/vm.rst |  5 ++--
>  mm/oom_kill.c                           | 31 +++++++++++++++++++++----
>  2 files changed, 30 insertions(+), 6 deletions(-)
> 
> diff --git a/Documentation/admin-guide/sysctl/vm.rst b/Documentation/admin-guide/sysctl/vm.rst
> index 003d5cc3751b..4c79fa00ddb3 100644
> --- a/Documentation/admin-guide/sysctl/vm.rst
> +++ b/Documentation/admin-guide/sysctl/vm.rst
> @@ -650,8 +650,9 @@ oom_dump_tasks
>  Enables a system-wide task dump (excluding kernel threads) to be produced
>  when the kernel performs an OOM-killing and includes such information as
>  pid, uid, tgid, vm size, rss, pgtables_bytes, swapents, oom_score_adj
> -score, and name.  This is helpful to determine why the OOM killer was
> -invoked, to identify the rogue task that caused it, and to determine why
> +score, oom eligibility status and name.  This is helpful to determine why
> +the OOM killer was invoked, to identify the rogue task that caused it, and
> +to determine why
>  the OOM killer chose the task it did to kill.
>  
>  If this is set to zero, this information is suppressed.  On very
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index c729a4c4a1ac..36daa6917b62 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 * const is_task_oom_eligible(struct task_struct *p)
> +{
> +	long adj;
> +
> +	adj = (long)p->signal->oom_score_adj;
> +	if (adj == OOM_SCORE_ADJ_MIN)
> +		return "M";

oom_score_adj is shown already in the tasklist dump, I'm not sure what 
value this adds.

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

This is going to be racy, we can't show that a task that is emitted as 
part of the tasklist dump was did not have in_vfork() == true at the time 
oom_badness() was called.

Wouldn't it be better to simply print the output of oom_badness() to the 
tasklist dump instead so we get complete information?

We could simply special case a LONG_MIN return value as -1000 or "min".

> +	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 %1s %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->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_skipped name\n");
>  
>  	if (is_memcg_oom(oc))
>  		mem_cgroup_scan_tasks(oc->memcg, dump_task, oc);
> -- 
> 2.31.1
> 
>