linux-kernel - Re: Re: Re: [PATCH 4/13] memcg: force

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Date:	Tue, 23 Sep 2008 00:06:10 +0900 (JST)
From:	kamezawa.hiroyu@...fujitsu.com
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	kamezawa.hiroyu@...fujitsu.com, linux-mm@...ck.org,
	balbir@...ux.vnet.ibm.com, nishimura@....nes.nec.co.jp,
	xemul@...nvz.org, LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: Re: Re: [PATCH 4/13] memcg: force_empty moving account

----- Original Message -----
>On Mon, 2008-09-22 at 23:50 +0900, kamezawa.hiroyu@...fujitsu.com wrote:
>> ----- Original Message -----
>> >> +			spin_lock_irqsave(&mz->lru_lock, flags);
>> >> +		} else {
>> >> +			unlock_page(page);
>> >> +			put_page(page);
>> >> +		}
>> >> +		if (atomic_read(&mem->css.cgroup->count) > 0)
>> >> +			break;
>> >>  	}
>> >>  	spin_unlock_irqrestore(&mz->lru_lock, flags);
>> >
>> >do _NOT_ use yield() ever! unless you know what you're doing, and
>> >probably not even then.
>> >
>> >NAK!
>> Hmm, sorry. cond_resched() is ok ?
>
>depends on what you want to do, please explain what you're trying to do.
>
Sorry again.

This force_empty is called only in following situation
 - there is no user threas in this cgroup.
 - a user tries to rmdir() this cgroup or explicitly type
   echo 1 > ../memory.force_empty.

force_empty() scans lru list of this cgroup and check page_cgroup on the
list one by one. Because there are no tasks in this group, force_empty can
see following racy condtions while scanning.

 - global lru tries to remove the page which pointed by page_cgroup 
   and it is not-on-LRU.
 - the page is locked by someone.
   ....find some lock contetion with invalidation/truncate.
 - in later patch, page_cgroup can be on pagevec(i added) and we have to drain
   it to remove from LRU.

In above situation, force_empty() have to wait for some event proceeds.

Hmm...detecting busy situation in loop and sleep in out-side-of-loop
is better ? Anyway, ok, I'll rewrite this.

BTW, sched.c::yield() is for what purpose now ?

Thanks,
-Kame



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