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

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Date:   Tue, 15 Jun 2021 13:02:57 +0100
From:   Aaron Tomlin <atomlin@...hat.com>
To:     David Rientjes <rientjes@...gle.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 Sun 2021-06-13 16:47 -0700, David Rientjes wrote:
> 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.

I see, sure.

> > +{
> > +	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.

I understand and perhaps it does not make sense; albeit, the reader might
not understand the meaning of OOM_SCORE_ADJ_MIN.

> > +	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?

I'd prefer not to show such tasks, when displaying potential OOM victims
(including those in_vfork()) as in my opinion, it can be misleading to the
reader. That said, a case has been made to maintain their inclusion.
However, should in_vfork() even be shown in the actual report?

> 
> > @@ -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?

This was to maintain some alignment but fair enough.

> >  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.

Understood.



Kind regards,

-- 
Aaron Tomlin