Message-ID: <alpine.DEB.2.00.0910230229010.28109@chino.kir.corp.google.com>
Date:	Fri, 23 Oct 2009 02:36:53 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Mel Gorman <mel@....ul.ie>
cc:	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>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	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 4/5] page allocator: Pre-emptively wake kswapd when
 high-order watermarks are hit

On Fri, 23 Oct 2009, Mel Gorman wrote:

> > Hmm, is this really supposed to be added to __alloc_pages_high_priority()?  
> > By the patch description I was expecting kswapd to be woken up 
> > preemptively whenever the preferred zone is below ALLOC_WMARK_LOW and 
> > we're known to have just allocated at a higher order, not just when 
> > current was oom killed (when we should already be freeing a _lot_ of 
> > memory soon) or is doing a higher order allocation during direct reclaim.
> > 
> 
> It was a somewhat arbitrary choice to have it trigger in the event high
> priority allocations were happening frequently.
> 

I don't quite understand, users of PF_MEMALLOC shouldn't be doing these 
higher order allocations and if ALLOC_NO_WATERMARKS is by way of the oom 
killer, we should be freeing a substantial amount of memory imminently 
when it exits that waking up kswapd would be irrelevant.

> > If this is moved to the fastpath, why is this wake_all_kswapd() and not
> > wakeup_kswapd(preferred_zone, order)?  Do we need to kick kswapd in all 
> > zones even though they may be free just because preferred_zone is now 
> > below the watermark?
> > 
> 
> It probably makes no difference as zones are checked for their watermarks
> before any real work happens. However, even if this patch makes a difference,
> I don't want to see it merged.  At best, it is an extremely heavy-handed
> hack which is why I asked for it to be tested in isolation. It shouldn't
> be necessary at all because sort of pre-emptive waking of kswapd was never
> necessary before.
> 

Ahh, that makes a ton more sense: this particular patch is a debugging 
effort while the first two are candidates for 2.6.32 and -stable.  Gotcha.

> > Wouldn't it be better to do this on page_zone(page) instead of 
> > preferred_zone anyway?
> > 
> 
> No. The preferred_zone is the zone we should be allocating from. If we
> failed to allocate from it, it implies the watermarks are not being met
> so we want to wake it.
> 

Oops, I'm even more confused now :)  I thought the existing 
wake_all_kswapd() in the slowpath was doing that and that this patch was 
waking them prematurely because it speculates that a subsequent high 
order allocation will fail unless memory is reclaimed.  I thought we'd  
want to reclaim from the zone we just did a high order allocation from so 
that the fastpath could find the memory next time with ALLOC_WMARK_LOW.
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