Re: [PATCH 1/4] oom: move oom_adj to signal_struct

[KOSAKI Motohiro]

> On Wed,  5 Aug 2009 11:29:34 +0900 (JST)
> KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx> 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. 

No. It's very old problem. David's patch fixed it. 
It mean per-process oom_adj prevent select_bad_process() return
a task in unkillable process.

unfortunatelly, his patch can't treat vfork case ideally. I hope to
fix 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 ?

prior Davied patch (task->oom_adj) might makes livelock.



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