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

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

On Wed, Dec 1, 2010 at 12:08 PM, Shaohua Li <shaohua.li@intel.com> 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>
Reviewed-by: Minchan Kim <minchan.kim@gmai.com>

But you need the description more easily.

I try it.

T0 : Task 1 wakes up kswapd with order-3
T1 : So, kswapd starts to reclaim pages using balance_pgdat
T2:  Task 2 wakes up kswapd with order-2 because pages reclaimed by T1
are consumed quickly.
T3: kswapd exits balance_pgdat and will recheck remained work
T4-1 : In beginning of kswapd's loop, pgdat->kswapd_max_order will be
reset with zero.
T4-2: If previous balance_pgdat can't meet requirement of order-3 free
pages by high watermark, it can start reclaiming again.
T4-3 :Unfortunately, balance_pgdat's argument _order_ is
pgdat->kswapd_max_order which was zero.  It should have been 2.

Regardless of my suggestion, I will add my Reviewed-by.

>
> 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);
>
>
>
> --
> 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>
>



-- 
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>