On Thu, 2010-12-02 at 09:23 +0800, Minchan Kim 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. > > 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. ok, I have no objection now. Reviewed-by: Shaohua Li <shaohua.li@xxxxxxxxx> -- 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>