Re: [PATCH 2/4] oom: kill younger process first

[Minchan Kim <minchan.kim@gmail.com>]

On Thu, May 12, 2011 at 10:53 AM, KAMEZAWA Hiroyuki
<kamezawa.hiroyu@jp.fujitsu.com> wrote:
> On Thu, 12 May 2011 10:30:45 +0900
> Minchan Kim <minchan.kim@gmail.com> wrote:
>
>> Hi Kame,
>>
>> On Thu, May 12, 2011 at 9:52 AM, KAMEZAWA Hiroyuki
>> <kamezawa.hiroyu@jp.fujitsu.com> wrote:
>> > On Tue, 10 May 2011 17:15:01 +0900 (JST)
>> > KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> wrote:
>> >
>> >> This patch introduces do_each_thread_reverse() and
>> >> select_bad_process() uses it. The benefits are two,
>> >> 1) oom-killer can kill younger process than older if
>> >> they have a same oom score. Usually younger process
>> >> is less important. 2) younger task often have PF_EXITING
>> >> because shell script makes a lot of short lived processes.
>> >> Reverse order search can detect it faster.
>> >>
>> >> Reported-by: CAI Qian <caiqian@redhat.com>
>> >> Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
>> >
>> > IIUC, for_each_thread() can be called under rcu_read_lock() but
>> > for_each_thread_reverse() must be under tasklist_lock.
>>
>> Just out of curiosity.
>> You mentioned it when I sent forkbomb killer patch. :)
>> From at that time, I can't understand why we need holding
>> tasklist_lock not rcu_read_lock. Sorry for the dumb question.
>>
>> At present, it seems that someone uses tasklist_lock and others uses
>> rcu_read_lock. But I can't find any rule for that.
>>
>
> for_each_list_rcu() makes use of RCU list's characteristics and allows
> walk a list under rcu_read_lock() without taking any atomic locks.
>
> list_del() of RCU list works as folllowing.
>
> ==
>  1) assume  A, B, C, are linked in the list.
>        (head)<->(A) <-> (B)  <-> (C)
>
>  2) remove B.
>        (head)<->(A) <-> (C)
>                        /
>                     (B)
>
>  Because (B)'s next points to (C) even after (B) is removed, (B)->next
>  points to the alive object. Even if (C) is removed at the same time,
>  (C) is not freed until rcu glace period and (C)'s next points to (head)
>
> Then, for_each_list_rcu() can work well under rcu_read_lock(), it will visit
> only alive objects (but may not be valid.)
>
> ==
>
> please see include/linux/rculist.h and check list_add_rcu() ;)
>
> As above implies, (B)->prev pointer is invalid pointer after list_del().
> So, there will be race with list modification and for_each_list_reverse under
> rcu_read__lock()
>
> So, when you need to take atomic lock (as tasklist lock is) is...
>
>  1) You can't check 'entry' is valid or not...
>    In above for_each_list_rcu(), you may visit an object which is under removing.
>    You need some flag or check to see the object is valid or not.
>
>  2) you want to use list_for_each_safe().
>    You can't do list_del() an object which is under removing...
>
>  3) You want to walk the list in reverse.
>
>  3) Some other reasons. For example, you'll access an object pointed by the
>    'entry' and the object is not rcu safe.
>
> make sense ?

Yes. Thanks, Kame.
It seems It is caused by prev poisoning of list_del_rcu.
If we remove it, isn't it possible to traverse reverse without atomic lock?



>
> Thanks,
> -Kame
>
>
>> Could you elaborate it, please?
>> Doesn't it need document about it?
>>
>> --
>> Kind regards,
>> Minchan Kim
>>
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>
>



-- 
Kind regards,
Minchan Kim

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