On Wed, Jun 27, 2012 at 1:21 PM, Minchan Kim <minchan@xxxxxxxxxx> wrote: > When hotplug offlining happens on zone A, it starts to mark freed page > as MIGRATE_ISOLATE type in buddy for preventing further allocation. > (MIGRATE_ISOLATE is very irony type because it's apparently on buddy > but we can't allocate them). > When the memory shortage happens during hotplug offlining, > current task starts to reclaim, then wake up kswapd. > Kswapd checks watermark, then go sleep because current zone_watermark_ok_safe > doesn't consider MIGRATE_ISOLATE freed page count. > Current task continue to reclaim in direct reclaim path without kswapd's helping. > The problem is that zone->all_unreclaimable is set by only kswapd > so that current task would be looping forever like below. > > __alloc_pages_slowpath > restart: > wake_all_kswapd > rebalance: > __alloc_pages_direct_reclaim > do_try_to_free_pages > if global_reclaim && !all_unreclaimable > return 1; /* It means we did did_some_progress */ > skip __alloc_pages_may_oom > should_alloc_retry > goto rebalance; > > If we apply KOSAKI's patch[1] which doesn't depends on kswapd > about setting zone->all_unreclaimable, we can solve this problem > by killing some task in direct reclaim path. But it doesn't wake up kswapd, still. > It could be a problem still if other subsystem needs GFP_ATOMIC request. > So kswapd should consider MIGRATE_ISOLATE when it calculate free pages > BEFORE going sleep. > > This patch counts the number of MIGRATE_ISOLATE page block and > zone_watermark_ok_safe will consider it if the system has such blocks > (fortunately, it's very rare so no problem in POV overhead and kswapd is never > hotpath). > > Copy/modify from Mel's quote > " > Ideal solution would be "allocating" the pageblock. > It would keep the free space accounting as it is but historically, > memory hotplug didn't allocate pages because it would be difficult to > detect if a pageblock was isolated or if part of some balloon. > Allocating just full pageblocks would work around this, However, > it would play very badly with CMA. > " > > [1] http://lkml.org/lkml/2012/6/14/74 > > * from v1 > - add changelog > - make functions simple > - remove atomic variable > - discard exact isolated free page accounting. > - rebased on next-20120626 > > Suggested-by: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx> > Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> > Cc: Aaditya Kumar <aaditya.kumar.30@xxxxxxxxx> > Cc: Mel Gorman <mgorman@xxxxxxx> > Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx> > --- > > Aaditya, coul you confirm this patch solve your problem and > make sure nr_pageblock_isolate is zero after hotplug end? I am really sorry for the delay. I just tried this patch on my ARM setup. > > +#ifdef CONFIG_MEMORY_ISOLATION > +static inline unsigned long nr_zone_isolate_freepages(struct zone *zone) > +{ > + unsigned long nr_pages = 0; > + > + if (unlikely(zone->nr_pageblock_isolate)) { > + nr_pages = zone->nr_pageblock_isolate * pageblock_nr_pages; > + } > + return nr_pages; > +} > +#else > +static inline unsigned long nr_zone_isolate_freepages(struct zone *zone) > +{ > + return 0; > +} > +#endif > + > bool zone_watermark_ok(struct zone *z, int order, unsigned long mark, > int classzone_idx, int alloc_flags) > { > @@ -1629,6 +1651,14 @@ bool zone_watermark_ok_safe(struct zone *z, int order, unsigned long mark, > if (z->percpu_drift_mark && free_pages < z->percpu_drift_mark) > free_pages = zone_page_state_snapshot(z, NR_FREE_PAGES); > > + /* > + * If the zone has MIGRATE_ISOLATE type free page, > + * we should consider it. nr_zone_isolate_freepages is never > + * accurate so kswapd might not sleep although she can. > + * But it's more desirable for memory hotplug rather than > + * forever sleep which cause livelock in direct reclaim path. > + */ > + free_pages -= nr_zone_isolate_freepages(z); > return __zone_watermark_ok(z, order, mark, classzone_idx, alloc_flags, > free_pages); For my test case, pages to be off lined span the whole node. With this setup the free_pages become negative. (As you and Kamezawa-san already expected.) BUT because of free_pages going negative the memory off lining still livelocks as __zone_watermark_ok() returns true. This is because in below if comparison, because of an unsigned value (z->lowmem_reserve[classzone_idx]) all the longs are converted to unsigned long. static bool __zone_watermark_ok(struct zone *z, int order, unsigned long mark, int classzone_idx, int alloc_flags, long free_pages) { <snip> if (free_pages <= min + z->lowmem_reserve[classzone_idx]) return false; <snip> So, may be you can consider following also: As for the nr_pageblock_isolate going back to zero, yes it is going back to zero for my test case.(I tested after this change) --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -1594,6 +1594,7 @@ static bool __zone_watermark_ok(struct zone *z, int order, unsigned long mark, { /* free_pages my go negative - that's OK */ long min = mark; + long lowmem_res = z->lowmem_reserve[classzone_idx]; int o; free_pages -= (1 << order) - 1; @@ -1602,7 +1603,7 @@ static bool __zone_watermark_ok(struct zone *z, int order, unsigned long mark, if (alloc_flags & ALLOC_HARDER) min -= min / 4; - if (free_pages <= min + z->lowmem_reserve[classzone_idx]) + if (free_pages <= min + lowmem_res) return false; -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. 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