Re: [PATCH RFC] mm/memcg: recalculate chargeable space after waiting migrating charges

[Wanpeng Li <liwp.linux-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

On Thu, Jul 12, 2012 at 02:29:38PM +0200, Michal Hocko wrote:
>On Thu 12-07-12 19:51:25, Wanpeng Li wrote:
>> On Thu, Jul 12, 2012 at 01:08:38PM +0200, Michal Hocko wrote:
>> >On Thu 12-07-12 18:39:21, Wanpeng Li wrote:
>> >> From: Wanpeng Li <liwp-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org>
>> >> 
>> >> Function mem_cgroup_do_charge will call mem_cgroup_reclaim,
>> >> there are two break points in mem_cgroup_reclaim:
>> >> if (total && (flag & MEM_CGROUP_RECLAIM_SHIRINK))
>> >> 	break;
>> >> if (mem_cgroup_margin(memcg))
>> >> 	break;
>> >> so mem_cgroup_reclaim can't guarantee reclaim enough pages(nr_pages) 
>> >> which is requested from mem_cgroup_do_charge, if mem_cgroup_margin
>> >> (mem_over_limit) >= nr_pages is not true, the process will go to
>> >> mem_cgroup_wait_acct_move to wait doubly charge counted caused by
>> >> task move. 
>> >
>> >I am sorry but I have no idea what you are trying to say. The
>> >mem_cgroup_wait_acct_move just makes sure that we are waiting until
>> >charge is moved (which can potentially free some charges) rather than
>> >OOM which should be the last resort so it makes sense to retry them
>> >charge.
>> >
>> >> But this time still can't guarantee enough pages(nr_pages) is
>> >> ready, directly return CHARGE_RETRY is incorret. 
>> >
>> >So you think it is better to oom? Why? What prevents you from a race
>> >that your mem_cgroup_margin returns true but another CPU consumes those
>> >charges right after that. See? The check is pointless. It doesn't
>> 
>> Hmm, if there are a race as you mentioned it can't guarantee enough pages 
>> is ready. 
>
>And there is no point in guaranteeing anything which I tried to tell you
>by the example... The only thing that matters is whether we get the charge
>on the next attempt and if not whether we are able to reclaim something.
>See?
>
>> But it also means that available memory is too low if this
>> race happen. If available charges still less than nr_pages
>> after mem_cgroup_wait_acct_move(which can potentially
>> free some charges) return, the CHAGE_RETRY will trigged,
>> and then mem_cgroup_do_charge=>meory_cgroup_reclaim
>> =>mem_cgroup_wait_acct_move, if available charges still less than
>> nr_pages in this round, CHAGE_RETRY.....
>
>> To avoid this infinite retry when available memory 
>
>I do not see a realistic scenario which would cause this to be infinite loop
>withou OOM jumping in.
>We would have to hit the wait for move after each reclaim and the move would
>have to keep the the usage constant (move is really fast without moving
>charges).
>So what you are trying to address (if I understand it at all) is to fix
>an almost impossible to trigger issue with a bogus change which doesn't
>help at all because it is racy as well.

OK. Thank you Michal! :-)

Thanks & Best Regards,
Wanpeng Li

>
>> in this memcg is very low, go to OOM if mem_cgroup_margin(mem_over_limit) 
>> < nr_pages is a better way I think. Because the codes have already try
>> its best to reclaim some pages. :-)
>
>
>> 
>[...]
>-- 
>Michal Hocko
>SUSE Labs
>SUSE LINUX s.r.o.
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>190 00 Praha 9    
>Czech Republic