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SubjectRe: [PATCH 1/4] oom: move oom_adj to signal_struct
On Wed,  5 Aug 2009 11:29:34 +0900 (JST)
KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> wrote:

> Hi
>
> > Hi, Kosaki.
> >
> > I am so late to invole this thread.
> > But let me have a question.
> >
> > What's advantage of placing oom_adj in singal rather than task ?
> > I mean task->oom_adj and task->signal->oom_adj ?
> >
> > I am sorry if you already discussed it at last threads.
>
> Not sorry. that's very good question.
>
> I'm trying to explain the detailed intention of commit 2ff05b2b4eac
> (move oom_adj to mm_struct).
>
> In 2.6.30, OOM logic callflow is here.
>
> __out_of_memory
> select_bad_process for each task
> badness calculate badness of one task
> oom_kill_process search child
> oom_kill_task kill target task and mm shared tasks with it
>
> example, process-A have two thread, thread-A and thread-B and it
> have very fat memory.
> And, each thread have following likes oom property.
>
> thread-A: oom_adj = OOM_DISABLE, oom_score = 0
> thread-B: oom_adj = 0, oom_score = very-high
>
> Then, select_bad_process() select thread-B, but oom_kill_task refuse
> kill the task because thread-A have OOM_DISABLE.
> __out_of_memory() call select_bad_process() again. but select_bad_process()
> select the same task. It mean kernel fall in the livelock.
>
> The fact is, select_bad_process() must select killable task. otherwise
> OOM logic go into livelock.
>
> Is this enough explanation? thanks.
>

Thanks for good explanation. :)

It resulted from patch of David which moved task_struct->oom_ajd
to mm_struct. I understood it.

It meant oom_adj was not per-process.

AFAIU, you want to make oom_adj per-process, again.
And you selected the place with task->singal as per-process.

What I have a question is that why do you select task_struct->signal
rather than task_struct like old?

What's benefit of using task_struct->signal ?


--
Kind regards,
Minchan Kim


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