linux-kernel - Re: [patch -mm v2] mm: introduce oom_adj

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Date:	Fri, 31 Jul 2009 12:18:42 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>,
	Paul Menage <menage@...gle.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [patch -mm v2] mm: introduce oom_adj_child

On Fri, 31 Jul 2009, KAMEZAWA Hiroyuki wrote:

> > Can you help think of any names that start with oom_adj_* and are
> > relatively short?  I'd happily ack it.
> >
> There have been traditional name "effective" as uid and euid.
> 
>  then,  per thread oom_adj as oom_adj
>         per proc   oom_adj as effective_oom_adj
> 
> is an natural way as Unix, I think.
> 

I don't think effective_oom_adj is a suitable name replacement for 
oom_adj_child since it doesn't imply that the value is a no-op for the 
thread itself and only serves a purpose when an mm is initialized for a 
child.

> > It livelocks if a thread is chosen and passed to oom_kill_task() while
> > another per-thread oom_adj value is OOM_DISABLE for a thread sharing the
> > same memory.
> >
> I say "why don't modify buggy selection logic?"
> 
> Why we have to scan all threads ?
> As fs/proc/readdir does, you can scan only "process group leader".
> 
> per-thread scan itself is buggy because now we have per-process
> effective-oom-adj.
> 

Without my patches to change oom_adj from task_struct to mm_struct, you'd 
need to scan all tasks and not just the tgids because their oom_adj values 
can differ amongst threads in the same thread group.  So while it may now 
be possible to shorten the scan as a result of my approach, it isn't a 
solution itself to the problem.

> > How else do you propose the oom killer use oom_adj values on a per-thread
> > basis without considering other threads sharing the same memory?
> As I wrote.
>    per-process(signal struct) or per-thread oom_adj and add
>    mm->effecitve_oom_adj
> 
> task scanning isn't necessary to do per-thread scan and you can scan
> only process-group-leader. What's bad ?
> If oom_score is problem, plz fix it to show effective_oom_score.
> 

When only using (and showing) mm->effective_oom_adj for a task, userspace 
will not be able to adjust /proc/pid/oom_score with /proc/pid/oom_adj 
as Documentation/filesystems/proc.txt says you can for a thread unless it 
exceeds effective_oom_adj.
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