netdev - Re: [PATCH 1/5] page allocator: Always wake kswapd when restarting an allocation attempt after direct reclaim failed

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Date:	Tue, 27 Oct 2009 11:42:55 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	David Rientjes <rientjes@...gle.com>
Cc:	kosaki.motohiro@...fujitsu.com, Mel Gorman <mel@....ul.ie>,
	Frans Pop <elendil@...net.nl>, Jiri Kosina <jkosina@...e.cz>,
	Sven Geggus <lists@...hsschwanzdomain.de>,
	Karol Lewandowski <karol.k.lewandowski@...il.com>,
	Tobias Oetiker <tobi@...iker.ch>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	David Miller <davem@...emloft.net>,
	Reinette Chatre <reinette.chatre@...el.com>,
	Kalle Valo <kalle.valo@....fi>,
	Mohamed Abbas <mohamed.abbas@...el.com>,
	Jens Axboe <jens.axboe@...cle.com>,
	"John W. Linville" <linville@...driver.com>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Bartlomiej Zolnierkiewicz <bzolnier@...il.com>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Stephan von Krawczynski <skraw@...net.com>,
	Kernel Testers List <kernel-testers@...r.kernel.org>,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	"linux-mm@...ck.org\"" <linux-mm@...ck.org>
Subject: Re: [PATCH 1/5] page allocator: Always wake kswapd when restarting an allocation attempt after direct reclaim failed

> On Mon, 26 Oct 2009, KOSAKI Motohiro wrote:
> 
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index bf72055..5a27896 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -1899,6 +1899,12 @@ rebalance:
> >  	if (should_alloc_retry(gfp_mask, order, pages_reclaimed)) {
> >  		/* Wait for some write requests to complete then retry */
> >  		congestion_wait(BLK_RW_ASYNC, HZ/50);
> > +
> > +		/*
> > +		 * While we wait congestion wait, Amount of free memory can
> > +		 * be changed dramatically. Thus, we kick kswapd again.
> > +		 */
> > +		wake_all_kswapd(order, zonelist, high_zoneidx);
> >  		goto rebalance;
> >  	}
> >  
> 
> We're blocking to finish writeback of the directly reclaimed memory, why 
> do we need to wake kswapd afterwards?

the same reason of "goto restart" case. that's my intention.
if following scenario occur, it is equivalent that we didn't call wake_all_kswapd().

  1. call congestion_wait()
  2. kswapd reclaimed lots memory and sleep
  3. another task consume lots memory
  4. wakeup from congestion_wait()

IOW, if we falled into __alloc_pages_slowpath(), we naturally expect
next page_alloc() don't fall into slowpath. however if kswapd end to
its work too early, this assumption isn't true.

Is this too pessimistic assumption?





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