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    SubjectRe: [PATCH 0/3] Reduce watermark-related problems with the per-cpu allocator V4
    On Fri,  3 Sep 2010 10:08:43 +0100
    Mel Gorman <> 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.


    > 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.

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