linux-kernel - Re: [PATCH] mm,oom: Exclude TIF_MEMDIE processes from candidates.

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Date:	Mon, 11 Jan 2016 16:18:35 +0100
From:	Michal Hocko <mhocko@...nel.org>
To:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc:	rientjes@...gle.com, akpm@...ux-foundation.org, mgorman@...e.de,
	torvalds@...ux-foundation.org, oleg@...hat.com, hughd@...gle.com,
	andrea@...nel.org, riel@...hat.com, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm,oom: Exclude TIF_MEMDIE processes from candidates.

On Fri 08-01-16 00:38:43, Tetsuo Handa wrote:
> Michal Hocko wrote:
> > @@ -333,6 +333,14 @@ static struct task_struct *select_bad_process(struct oom_control *oc,
> >  		if (points == chosen_points && thread_group_leader(chosen))
> >  			continue;
> >  
> > +		/*
> > +		 * If the current major task is already ooom killed and this
> > +		 * is sysrq+f request then we rather choose somebody else
> > +		 * because the current oom victim might be stuck.
> > +		 */
> > +		if (is_sysrq_oom(sc) && test_tsk_thread_flag(p, TIF_MEMDIE))
> > +			continue;
> > +
> >  		chosen = p;
> >  		chosen_points = points;
> >  	}
> 
> Do we want to require SysRq-f for each thread in a process?
> If g has 1024 p, dump_tasks() will do
> 
>   pr_info("[%5d] %5d %5d %8lu %8lu %7ld %7ld %8lu         %5hd %s\n",
> 
> for 1024 times? I think one SysRq-f per one process is sufficient.

I am not following you here. If we kill the process the whole process
group (aka all threads) will get killed which ever thread we happen to
send the sigkill to.
 
> How can we guarantee that find_lock_task_mm() from oom_kill_process()
> chooses !TIF_MEMDIE thread when try_to_sacrifice_child() somehow chose
> !TIF_MEMDIE thread? I think choosing !TIF_MEMDIE thread at
> find_lock_task_mm() is the simplest way.

find_lock_task_mm chosing TIF_MEMDIE thread shouldn't change anything
because the whole thread group will go down anyway. If you want to
guarantee that the sysrq+f never choses a task which has a TIF_MEMDIE
thread then we would have to check for fatal_signal_pending as well
AFAIU. Fiddling with find find_lock_task_mm will not help you though
unless I am missing something.
-- 
Michal Hocko
SUSE Labs