Re: [patch]vmscan: make kswapd use a correct order

[Shaohua Li <shaohua.li@intel.com>]

On Thu, 2010-12-02 at 08:54 +0800, Minchan Kim wrote:
> On Thu, Dec 2, 2010 at 9:29 AM, Shaohua Li <shaohua.li@intel.com> wrote:
> > On Wed, 2010-12-01 at 23:58 +0800, Minchan Kim wrote:
> >> On Wed, Dec 01, 2010 at 06:44:27PM +0900, KOSAKI Motohiro wrote:
> >> > > T0: Task1 wakeup_kswapd(order=3)
> >> > > T1: kswapd enters balance_pgdat
> >> > > T2: Task2 wakeup_kswapd(order=2), because pages reclaimed by kswapd are used
> >> > > quickly
> >> > > T3: kswapd exits balance_pgdat. kswapd will do check. Now new order=2,
> >> > > pgdat->kswapd_max_order will become 0, but order=3, if sleeping_prematurely,
> >> > > then order will become pgdat->kswapd_max_order(0), while at this time the
> >> > > order should 2
> >> > > This isn't a big deal, but we do have a small window the order is wrong.
> >> > >
> >> > > Signed-off-by: Shaohua Li <shaohua.li@intel.com>
> >> > >
> >> > > diff --git a/mm/vmscan.c b/mm/vmscan.c
> >> > > index d31d7ce..15cd0d2 100644
> >> > > --- a/mm/vmscan.c
> >> > > +++ b/mm/vmscan.c
> >> > > @@ -2450,7 +2450,7 @@ static int kswapd(void *p)
> >> > >                           }
> >> > >                   }
> >> > >
> >> > > -                 order = pgdat->kswapd_max_order;
> >> > > +                 order = max_t(unsigned long, new_order, pgdat->kswapd_max_order);
> >> > >           }
> >> > >           finish_wait(&pgdat->kswapd_wait, &wait);
> >> >
> >> > Good catch!
> >> >
> >> > But unfortunatelly, the code is not correct. At least, don't fit corrent
> >> > design.
> >> >
> >> > 1) if "order < new_order" condition is false, we already decided to don't
> >> >    use new_order. So, we shouldn't use new_order after kswapd_try_to_sleep()
> >> > 2) if sleeping_prematurely() return false, it probably mean
> >> >    zone_watermark_ok_safe(zone, order, high_wmark) return false.
> >> >    therefore, we have to retry reclaim by using old 'order' parameter.
> >>
> >> Good catch, too.
> >>
> >> In Shaohua's scenario, if Task1 gets the order-3 page after kswapd's reclaiming,
> >> it's no problem.
> >> But if Task1 doesn't get the order-3 page and others used the order-3 page for Task1,
> >> Kswapd have to reclaim order-3 for Task1, again.
> > why? it's just a possibility. Task1 might get its pages too. If Task1
> > doesn't get its pages, it will wakeup kswapd too with its order.
> >
> >> In addtion, new order is always less than old order in that context.
> >> so big order page reclaim makes much safe for low order pages.
> > big order page reclaim makes we have more chances to reclaim useful
> > pages by lumpy, why it's safe?
> 
> For example, It assume tat Task1 continues to fail get the order-3
> page of GFP_ATOMIC since other tasks continues to allocate order-2
> pages so that they steal pages. 
but even you reclaim order-3, you can't guarantee task1 can get the
pages too. order-3 page can be steal by order-2 allocation

> Then, your patch makes continue to
> reclaim order-2 page in this scenario. Task1 never get the order-3
> pages if it doesn't have a merge luck.
Task1 will wakeup kswapd again for order-3, so kswapd will reclaim
order-3 very soon after the order-2 reclaim.

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