linux-kernel - Re: [PATCH] OOM killer: wait for tasks with pending SIGKILL to exit

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Date:	Wed, 11 Sep 2013 19:06:05 +0400
From:	Sergey Dyasly <dserrg@...il.com>
To:	David Rientjes <rientjes@...gle.com>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Michal Hocko <mhocko@...e.cz>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Sha Zhengju <handai.szj@...bao.com>,
	Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH] OOM killer: wait for tasks with pending SIGKILL to exit

On Mon, 9 Sep 2013 13:07:08 -0700 (PDT)
David Rientjes <rientjes@...gle.com> wrote:

> >  		/*
> >  		 * If this task is not being ptraced on exit, then wait for it
> >  		 * to finish before killing some other task unnecessarily.
> >  		 */
> > -		if (!(task->group_leader->ptrace & PT_TRACE_EXIT))
> > +		if (!(task->group_leader->ptrace & PT_TRACE_EXIT)) {
> > +			set_tsk_thread_flag(task, TIF_MEMDIE);
> 
> This does not, we do not give access to memory reserves unless the process 
> needs it to allocate memory.  The task here, which is not current, can 
> call into the oom killer and be granted memory reserves if necessary.

True. However, why TIF_MEMDIE is set for PF_EXITING task in oom_kill_process()
then?
Also, setting TIF_MEMDIE will avoid direct reclaim and memory allocation should
be fast if exiting task needs it.

> > @@ -412,16 +415,6 @@ void oom_kill_process(struct task_struct *p, gfp_t gfp_mask, int order,
> >  	static DEFINE_RATELIMIT_STATE(oom_rs, DEFAULT_RATELIMIT_INTERVAL,
> >  					      DEFAULT_RATELIMIT_BURST);
> >  
> > -	/*
> > -	 * If the task is already exiting, don't alarm the sysadmin or kill
> > -	 * its children or threads, just set TIF_MEMDIE so it can die quickly
> > -	 */
> > -	if (p->flags & PF_EXITING) {
> > -		set_tsk_thread_flag(p, TIF_MEMDIE);
> > -		put_task_struct(p);
> > -		return;
> > -	}
> 
> I think you misunderstood the point of this; if a selected process is 
> already in the exit path then this is simply avoiding dumping oom kill 
> lines to the kernel log.  We want to keep doing that.

This happens in oom_kill_process() after victim has been selected by
select_bad_process(). But there is already PF_EXITING check in
oom_scan_process_thread() and in this case OOM code won't call oom_kill_process.
There is only a slight chance that victim will become PF_EXITING between
scan and kill.

The only difference is in force_kill flag, and the only case where it's set
is SysRq. And I think in this case OOM killer messages are a good thing to have
even when victim is already exiting, instead of just silence.

-- 
Sergey Dyasly <dserrg@...il.com>
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