linux-kernel - Re: [patch 1/2] mm: page_alloc: reset aging cycle with GFP

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Date:	Fri, 28 Feb 2014 11:45:01 +0000
From:	Mel Gorman <mgorman@...e.de>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Jan Stancek <jstancek@...hat.com>,
	Rik van Riel <riel@...hat.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [patch 1/2] mm: page_alloc: reset aging cycle with GFP_THISNODE

On Wed, Feb 26, 2014 at 03:13:33PM -0500, Johannes Weiner wrote:
> On Wed, Feb 26, 2014 at 12:12:06PM -0500, Johannes Weiner wrote:
> > On Wed, Feb 26, 2014 at 09:54:22AM +0000, Mel Gorman wrote:
> > > How about special casing the (alloc_flags & ALLOC_WMARK_LOW) check in
> > > get_page_from_freelist to also ignore GFP_THISNODE? The NR_ALLOC_BATCH
> > > will go further negative if there are storms of GFP_THISNODE allocations
> > > forcing other allocations into the slow path doing multiple calls to
> > > prepare_slowpath but it would be closer to current behaviour and avoid
> > > weirdness with kswapd.
> > 
> > I think the result would be much uglier.  The allocations wouldn't
> > participate in the fairness protocol, and they'd create work for
> > kswapd without waking it up, diminishing the latency reduction for
> > which we have kswapd in the first place.
> > 
> > If kswapd wakeups should be too aggressive, I'd rather we ratelimit
> > them in some way rather than exempting random order-0 allocation types
> > as a moderation measure.  Exempting higher order wakeups, like THP
> > does is one thing, but we want order-0 watermarks to be met at all
> > times anyway, so it would make sense to me to nudge kswapd for every
> > failing order-0 request.
> 
> So I'd still like to fix this and wake kswapd even for GFP_THISNODE
> allocations, but let's defer it for now in favor of a minimal bugfix
> that can be ported to -stable.
> 
> Would this be an acceptable replacement for 1/2?
> 
> ---
> 
> From: Johannes Weiner <hannes@...xchg.org>
> Subject: [patch 1/2] mm: page_alloc: exempt GFP_THISNODE allocations from zone
>  fairness
> 
> Jan Stancek reports manual page migration encountering allocation
> failures after some pages when there is still plenty of memory free,
> and bisected the problem down to 81c0a2bb515f ("mm: page_alloc: fair
> zone allocator policy").
> 
> The problem is that GFP_THISNODE obeys the zone fairness allocation
> batches on one hand, but doesn't reset them and wake kswapd on the
> other hand.  After a few of those allocations, the batches are
> exhausted and the allocations fail.
> 
> Fixing this means either having GFP_THISNODE wake up kswapd, or
> GFP_THISNODE not participating in zone fairness at all.  The latter
> seems safer as an acute bugfix, we can clean up later.
> 
> Reported-by: Jan Stancek <jstancek@...hat.com>
> Signed-off-by: Johannes Weiner <hannes@...xchg.org>
> Cc: <stable@...nel.org> # 3.12+

Acked-by: Mel Gorman <mgorman@...e.de>

-- 
Mel Gorman
SUSE Labs
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