linux-kernel - Re: [PATCH 4/8] Use prepare_to_wait_exclusive() instead prepare_to

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Date:	Tue, 15 Dec 2009 09:58:31 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Mike Galbraith <efault@....de>
CC:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	lwoodman@...hat.com, akpm@...ux-foundation.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, minchan.kim@...il.com
Subject: Re: [PATCH 4/8] Use prepare_to_wait_exclusive() instead prepare_to_wait()

On 12/15/2009 12:32 AM, Mike Galbraith wrote:
> On Tue, 2009-12-15 at 09:45 +0900, KOSAKI Motohiro wrote:
>>> On 12/14/2009 07:30 AM, KOSAKI Motohiro wrote:
>>>> if we don't use exclusive queue, wake_up() function wake _all_ waited
>>>> task. This is simply cpu wasting.
>>>>
>>>> Signed-off-by: KOSAKI Motohiro<kosaki.motohiro@...fujitsu.com>
>>>
>>>>    		if (zone_watermark_ok(zone, sc->order, low_wmark_pages(zone),
>>>>    					0, 0)) {
>>>> -			wake_up(wq);
>>>> +			wake_up_all(wq);
>>>>    			finish_wait(wq,&wait);
>>>>    			sc->nr_reclaimed += sc->nr_to_reclaim;
>>>>    			return -ERESTARTSYS;
>>>
>>> I believe we want to wake the processes up one at a time
>>> here.

>> Actually, wake_up() and wake_up_all() aren't different so much.
>> Although we use wake_up(), the task wake up next task before
>> try to alloate memory. then, it's similar to wake_up_all().

That is a good point.  Maybe processes need to wait a little
in this if() condition, before the wake_up().  That would give
the previous process a chance to allocate memory and we can
avoid waking up too many processes.

> What happens to waiters should running tasks not allocate for a while?

When a waiter is woken up, it will either:
1) see that there is enough free memory and wake up the next guy, or
2) run shrink_zone and wake up the next guy

Either way, the processes that just got woken up will ensure that
the sleepers behind them in the queue will get woken up.

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