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

[Minchan Kim <minchan.kim@gmail.com>]

Where is my mail?
I will resend lost content.

On Thu, Dec 2, 2010 at 10:23 AM, Minchan Kim <minchan.kim@gmail.com> wrote:
> On Thu, Dec 2, 2010 at 10:05 AM, Shaohua Li <shaohua.li@intel.com> wrote:
>> 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
>
> But at least, it has a high possibility to allocate order-3 page than
> reclaim order-2 pages.
>
>>
>>> 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.
>
> GFP_ATOMIC case doesn't wakeup kswapd.
> When kswapd wakeup by order-3 depends on caller's retry.
> And this situation can be repeated in next turn.
>
> We can't guarantee 100% Task-1's order-3 pages so I hope we should go
> way to reduce the problem as possible as we can.
>
>>
>>
>
>
>
> --
> Kind regards,
> Minchan Kim
>



-- 
Kind regards,
Minchan Kim

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Fight unfair telecom policy in Canada: sign http://dissolvethecrtc.ca/
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>