linux-kernel - Re: [PATCH 2/3] page allocator: Do not allow interrupts to use ALLOC

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Message-ID: <20091031222905.GA32720@elf.ucw.cz>
Date:	Sat, 31 Oct 2009 23:29:06 +0100
From:	Pavel Machek <pavel@....cz>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mel@....ul.ie>, stable@...nel.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	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>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Rik van Riel <riel@...hat.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Stephan von Krawczynski <skraw@...net.com>,
	kernel-testers@...r.kernel.org
Subject: Re: [PATCH 2/3] page allocator: Do not allow interrupts to use
 ALLOC_HARDER

On Sat 2009-10-31 14:19:50, David Rientjes wrote:
> On Sat, 31 Oct 2009, Pavel Machek wrote:
> 
> > > Um, no, it's a matter of the kernel implementation.  We allow such tasks 
> > > to allocate deeper into reserves to avoid the page allocator from 
> > > incurring a significant penalty when direct reclaim is required.  
> > > Background reclaim has already commenced at this point in the
> > > slowpath.
> > 
> > But we can't guarantee that enough memory will be ready in the
> > reserves. So if realtime task relies on it, it is broken, and will
> > fail to meet its deadlines from time to time.
> 
> This is truly a bizarre tangent to take, I don't quite understand the 
> point you're trying to make.  Memory reserves exist to prevent blocking 
> when we need memory the most (oom killed task or direct reclaim) and to 
> allocate from when we can't (GFP_ATOMIC) or shouldn't (rt tasks) utilize 
> direct reclaim.  The idea is to kick background reclaim first in the 
> slowpath so we're only below the low watermark for a short period and 
> allow the allocation to succeed.  If direct reclaim actually can't free 
> any memory, the oom killer will free it for us.
> 
> So the realtime[*] tasks aren't relying on it at all, the ALLOC_HARDER 
> exemption for them in the page allocator are a convenience to return 
> memory faster than otherwise when the fastpath fails.  I don't see much 
> point in arguing against that.

Well, you are trying to make rt heuristic more precise. I believe it
would be better to simply remove it.

-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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