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    SubjectRe: [PATCH 2/2] mm: page allocator: Do not drain per-cpu lists via IPI from page allocator context
    From
    On Thu, Jan 12, 2012 at 4:51 PM, Gilad Ben-Yossef <gilad@benyossef.com> wrote:
    > On Wed, Jan 11, 2012 at 12:11 PM, Mel Gorman <mgorman@suse.de> wrote:
    > <SNIP>
    >> Rather than making it safe to call get_online_cpus() from the page
    >> allocator, this patch simply removes the page allocator call to
    >> drain_all_pages(). To avoid impacting high-order allocation success
    >> rates, it still drains the local per-cpu lists for high-order
    >> allocations that failed. As a side effect, this reduces the number
    >> of IPIs sent during low memory situations.
    >>
    >> Signed-off-by: Mel Gorman <mgorman@suse.de>
    >> ---
    >>  mm/page_alloc.c |   16 ++++++++++++----
    >>  1 files changed, 12 insertions(+), 4 deletions(-)
    >>
    >> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
    >> index 2b8ba3a..b6df6fc 100644
    >> --- a/mm/page_alloc.c
    >> +++ b/mm/page_alloc.c
    >> @@ -1119,7 +1119,9 @@ void drain_local_pages(void *arg)
    >>  */
    >>  void drain_all_pages(void)
    >>  {
    >> +       get_online_cpus();
    >>        on_each_cpu(drain_local_pages, NULL, 1);
    >> +       put_online_cpus();
    >>  }
    >>
    >>  #ifdef CONFIG_HIBERNATION
    >> @@ -1982,11 +1984,17 @@ retry:
    >>                                        migratetype);
    >>
    >>        /*
    >> -        * If an allocation failed after direct reclaim, it could be because
    >> -        * pages are pinned on the per-cpu lists. Drain them and try again
    >> +        * If a high-order allocation failed after direct reclaim, there is a
    >> +        * possibility that it is because the necessary buddies have been
    >> +        * freed to the per-cpu list. Drain the local list and try again.
    >> +        * drain_all_pages is not used because it is unsafe to call
    >> +        * get_online_cpus from this context as it is possible that kthreadd
    >> +        * would block during thread creation and the cost of sending storms
    >> +        * of IPIs in low memory conditions is quite high.
    >>         */
    >> -       if (!page && !drained) {
    >> -               drain_all_pages();
    >> +       if (!page && order && !drained) {
    >> +               drain_pages(get_cpu());
    >> +               put_cpu();
    >>                drained = true;
    >>                goto retry;
    >>        }
    >> --
    >> 1.7.3.4
    >>
    >
    > I very much like the judo like quality of relying on the fact that in
    > memory pressure conditions most
    > of the cpus will end up in the direct reclaim path to drain them all
    > without IPIs.
    >
    > What I can't figure out is why we don't need  get/put_online_cpus()
    > pair around each and every call
    > to on_each_cpu everywhere? and if we do, perhaps making it a part of
    > on_each_cpu is the way to go?
    >
    > Something like:
    >
    > diff --git a/kernel/smp.c b/kernel/smp.c
    > index f66a1b2..cfa3882 100644
    > --- a/kernel/smp.c
    > +++ b/kernel/smp.c
    > @@ -691,11 +691,15 @@ void on_each_cpu(void (*func) (void *info), void
    > *info, int wait)
    >  {
    >        unsigned long flags;
    >
    > +       BUG_ON(in_atomic());
    > +
    > +       get_online_cpus();
    >        preempt_disable();
    >        smp_call_function(func, info, wait);
    >        local_irq_save(flags);
    >        func(info);
    >        local_irq_restore(flags);
    >        preempt_enable();
    > +       put_online_cpus();
    >  }
    >  EXPORT_SYMBOL(on_each_cpu);
    >
    > Does that makes?

    Well, it dies on boot (after adding the needed include file), so it's
    obviously wrong, but I'm still guessing
    other users of on_each_cpu will need an  get/put_online_cpus() wrapper too.

    Maybe -

    on_each_cpu(...)
    {
    get_online_cpus();
    __on_each_cpu(...);
    put_online_cpus();
    }

    We'll need to audit all callers.

    Gilad

    --
    Gilad Ben-Yossef
    Chief Coffee Drinker
    gilad@benyossef.com
    Israel Cell: +972-52-8260388
    US Cell: +1-973-8260388
    http://benyossef.com

    "Unfortunately, cache misses are an equal opportunity pain provider."
    -- Mike Galbraith, LKML
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