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

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Date:	Tue, 12 Jan 2016 20:52:00 +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 Tue 12-01-16 20:32:18, Tetsuo Handa wrote:
> Michal Hocko wrote:
> > 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.
> 
> Please distinguish "sending SIGKILL to a process" and "all threads in that
> process terminate".

I didn't say anything about termination if your read my response again.

[...]

> > > 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.
> 
> I do want to guarantee that the SysRq-f (and timeout based next victim
> selection) never chooses a process which has a TIF_MEMDIE thread.

Sigh... see what I have written in the paragraph you are replying to...

> I don't like current "oom: clear TIF_MEMDIE after oom_reaper managed to unmap
> the address space" patch unless both "mm,oom: exclude TIF_MEMDIE processes from
> candidates." patch and "mm,oom: Re-enable OOM killer using timers."

Those patches are definitely not a prerequisite from the functional
point of view and putting them together as a prerequisite sounds like
blocking a useful feature without technical grounds to me.

-- 
Michal Hocko
SUSE Labs