Re: Unending loop in __alloc_pages_slowpath following OOM-kill; rfc: patch.

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

(2011/05/24 18:16), Mel Gorman wrote:
> On Tue, May 24, 2011 at 06:05:59PM +0900, KOSAKI Motohiro wrote:
>>>>> Why?
>>>>
>>>> Otherwise, we don't have good PCP dropping trigger. Big machine might have
>>>> big pcp cache.
>>>>
>>>
>>> Big machines also have a large cost for sending IPIs.
>>
>> Yes. But it's only matter if IPIs are frequently happen.
>> But, drain_all_pages() is NOT only IPI source. some vmscan function (e.g.
>> try_to_umap) makes a lot of IPIs.
>>
>> Then, it's _relatively_ not costly. I have a question. Do you compare which
>> operation and drain_all_pages()? IOW, your "costly" mean which scenario suspect?
>>
> 
> I am concerned that if the machine gets into trouble and we are failing
> to reclaim that sending more IPIs is not going to help any. There is no
> evidence at the moment that sending extra IPIs here will help anything.

In old days, we always call drain_all_pages() if did_some_progress!=0. But
current kernel only call it when get_page_from_freelist() fail. So,
wait_iff_congested() may help but no guarantee to help us.

If you still strongly worry about IPI cost, I'm concern to move drain_all_pages()
to more unfrequently point. but to ignore pcp makes less sense, IMHO.



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