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SubjectRe: [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@csn.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@kernel.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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