On Thu, Dec 02, 2010 at 09:42:42AM +0000, Mel Gorman wrote: > On Thu, Dec 02, 2010 at 10:36:27AM +0900, Minchan Kim wrote: > > Where is my mail? > > I will resend lost content. > > > > On Thu, Dec 2, 2010 at 10:23 AM, Minchan Kim <minchan.kim@xxxxxxxxx> wrote: > > > On Thu, Dec 2, 2010 at 10:05 AM, Shaohua Li <shaohua.li@xxxxxxxxx> 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@xxxxxxxxx> 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@xxxxxxxxx> > > >>> >> > > > > >>> >> > > 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. > > > > > GFP_ATOMIC does wakeup kswapd. It just doesn't wait on kswapd to do > anything. I mean GFP_ATOMIC doesn't do restart for waking up kswapd again in __alloc_pages_slowpath. -- Kind regards, Minchan Kim -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx 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@xxxxxxxxx";> email@xxxxxxxxx </a>