Date:	Tue, 21 Sep 2010 12:17:41 +0100
From:	Mel Gorman <mel@....ul.ie>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Linux Kernel List <linux-kernel@...r.kernel.org>,
	linux-mm@...ck.org, Rik van Riel <riel@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Minchan Kim <minchan.kim@...il.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	stable@...nel.org, Dave Chinner <david@...morbit.com>
Subject: Re: [PATCH 0/3] Reduce watermark-related problems with the per-cpu
	allocator V4

On Fri, Sep 03, 2010 at 04:05:51PM -0700, Andrew Morton wrote:
> On Fri,  3 Sep 2010 10:08:43 +0100
> Mel Gorman <mel@....ul.ie> wrote:
> 
> > The noteworthy change is to patch 2 which now uses the generic
> > zone_page_state_snapshot() in zone_nr_free_pages(). Similar logic still
> > applies for *when* zone_page_state_snapshot() to avoid ovedhead.
> > 
> > Changelog since V3
> >   o Use generic helper for NR_FREE_PAGES estimate when necessary
> > 
> > Changelog since V2
> >   o Minor clarifications
> >   o Rebase to 2.6.36-rc3
> > 
> > Changelog since V1
> >   o Fix for !CONFIG_SMP
> >   o Correct spelling mistakes
> >   o Clarify a ChangeLog
> >   o Only check for counter drift on machines large enough for the counter
> >     drift to breach the min watermark when NR_FREE_PAGES report the low
> >     watermark is fine
> > 
> > Internal IBM test teams beta testing distribution kernels have reported
> > problems on machines with a large number of CPUs whereby page allocator
> > failure messages show huge differences between the nr_free_pages vmstat
> > counter and what is available on the buddy lists. In an extreme example,
> > nr_free_pages was above the min watermark but zero pages were on the buddy
> > lists allowing the system to potentially livelock unable to make forward
> > progress unless an allocation succeeds. There is no reason why the problems
> > would not affect mainline so the following series mitigates the problems
> > in the page allocator related to to per-cpu counter drift and lists.
> > 
> > The first patch ensures that counters are updated after pages are added to
> > free lists.
> > 
> > The second patch notes that the counter drift between nr_free_pages and what
> > is on the per-cpu lists can be very high. When memory is low and kswapd
> > is awake, the per-cpu counters are checked as well as reading the value
> > of NR_FREE_PAGES. This will slow the page allocator when memory is low and
> > kswapd is awake but it will be much harder to breach the min watermark and
> > potentially livelock the system.
> > 
> > The third patch notes that after direct-reclaim an allocation can
> > fail because the necessary pages are on the per-cpu lists. After a
> > direct-reclaim-and-allocation-failure, the per-cpu lists are drained and
> > a second attempt is made.
> > 
> > Performance tests against 2.6.36-rc3 did not show up anything interesting. A
> > version of this series that continually called vmstat_update() when
> > memory was low was tested internally and found to help the counter drift
> > problem. I described this during LSF/MM Summit and the potential for IPI
> > storms was frowned upon. An alternative fix is in patch two which uses
> > for_each_online_cpu() to read the vmstat deltas while memory is low and
> > kswapd is awake. This should be functionally similar.
> > 
> > This patch should be merged after the patch "vmstat : update
> > zone stat threshold at onlining a cpu" which is in mmotm as
> > vmstat-update-zone-stat-threshold-when-onlining-a-cpu.patch .
> > 
> > If we can agree on it, this series is a stable candidate.
> 
> (cc stable@...nel.org)
> 
> >  include/linux/mmzone.h |   13 +++++++++++++
> >  include/linux/vmstat.h |   22 ++++++++++++++++++++++
> >  mm/mmzone.c            |   21 +++++++++++++++++++++
> >  mm/page_alloc.c        |   29 +++++++++++++++++++++--------
> >  mm/vmstat.c            |   15 ++++++++++++++-
> >  5 files changed, 91 insertions(+), 9 deletions(-)
> 
> For the entire patch series I get
> 
>  include/linux/mmzone.h |   13 +++++++++++++
>  include/linux/vmstat.h |   22 ++++++++++++++++++++++
>  mm/mmzone.c            |   21 +++++++++++++++++++++
>  mm/page_alloc.c        |   33 +++++++++++++++++++++++----------
>  mm/vmstat.c            |   16 +++++++++++++++-
>  5 files changed, 94 insertions(+), 11 deletions(-)
> 
> The patches do apply OK to 2.6.35.
> 
> Give the extent and the coreness of it all, it's a bit more than I'd
> usually push at the -stable guys.  But I guess that if the patches fix
> all the issues you've noted, as well as David's "minute-long livelocks
> in memory reclaim" then yup, it's worth backporting it all.
> 

These patches have made it to mainline as the following commits.

9ee493c mm: page allocator: drain per-cpu lists after direct reclaim allocation fails
aa45484 mm: page allocator: calculate a better estimate of NR_FREE_PAGES when memory is low and kswapd is awake
72853e2 mm: page allocator: update free page counters after pages are placed on the free list

I have not heard from the -stable guys, is there a reasonable
expectation that they'll be picked up?

Thanks

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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