linux-kernel - Re: [RFC 1/3] oom, sysrq: Skip over oom victims and killed tasks

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Date:	Tue, 12 Jan 2016 16:41:50 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	Michal Hocko <mhocko@...nel.org>
cc:	linux-mm@...ck.org,
	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
	LKML <linux-kernel@...r.kernel.org>,
	Michal Hocko <mhocko@...e.com>
Subject: Re: [RFC 1/3] oom, sysrq: Skip over oom victims and killed tasks

On Tue, 12 Jan 2016, Michal Hocko wrote:

> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index abefeeb42504..2b9dc5129a89 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -326,6 +326,17 @@ static struct task_struct *select_bad_process(struct oom_control *oc,
>  		case OOM_SCAN_OK:
>  			break;
>  		};
> +
> +		/*
> +		 * If we are doing sysrq+f then it doesn't make any sense to
> +		 * check OOM victim or killed task because it might be stuck
> +		 * and unable to terminate while the forced OOM might be the
> +		 * only option left to get the system back to work.
> +		 */
> +		if (is_sysrq_oom(oc) && (test_tsk_thread_flag(p, TIF_MEMDIE) ||
> +				fatal_signal_pending(p)))
> +			continue;
> +
>  		points = oom_badness(p, NULL, oc->nodemask, totalpages);
>  		if (!points || points < chosen_points)
>  			continue;

I think you can make a case for testing TIF_MEMDIE here since there is no 
chance of a panic from the sysrq trigger.  However, I'm not convinced that 
checking fatal_signal_pending() is appropriate.  I think it would be 
better for sysrq+f to first select a process with fatal_signal_pending() 
set so it silently gets access to memory reserves and then a second 
sysrq+f to choose a different process, if necessary, because of 
TIF_MEMDIE.