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Message-ID: <20120713005710.GA5248@bbox>
Date: Fri, 13 Jul 2012 09:57:10 +0900
From: Minchan Kim <minchan@...nel.org>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
Mel Gorman <mgorman@...e.de>,
KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
Aaditya Kumar <aaditya.kumar@...sony.com>
Subject: Re: [PATCH 3/3 v3] memory-hotplug: fix kswapd looping forever problem
On Thu, Jul 12, 2012 at 02:01:54PM -0700, Andrew Morton wrote:
> On Thu, 12 Jul 2012 11:50:49 +0900
> Minchan Kim <minchan@...nel.org> 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
> >
> > ...
> >
> > +#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;
> > +}
>
> That's pretty verbose. I couldn't resist fiddling with it ;)
>
> > +#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)
> > {
> > @@ -1633,6 +1655,14 @@ 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.
> > + */
>
> I had a go at clarifying this comment. Please review the result.
>
> > + free_pages -= nr_zone_isolate_freepages(z);
> > return __zone_watermark_ok(z, order, mark, classzone_idx, alloc_flags,
> > free_pages);
> > }
> > @@ -4397,6 +4427,7 @@ static void __paginginit free_area_init_core(struct pglist_data *pgdat,
> > lruvec_init(&zone->lruvec, zone);
> > zap_zone_vm_stats(zone);
> > zone->flags = 0;
> > + zone->nr_pageblock_isolate = 0;
> > if (!size)
> > continue;
> >
> > diff --git a/mm/page_isolation.c b/mm/page_isolation.c
> > index fb482cf..64abb33 100644
> > --- a/mm/page_isolation.c
> > +++ b/mm/page_isolation.c
> > @@ -8,6 +8,31 @@
> > #include <linux/memory.h>
> > #include "internal.h"
> >
> > +/* called by holding zone->lock */
> > +static void set_pageblock_isolate(struct zone *zone, struct page *page)
> > +{
> > + BUG_ON(page_zone(page) != zone);
>
> Well. If this is the case then why not eliminate this function's
> `zone' argument?
>
> > + if (get_pageblock_migratetype(page) == MIGRATE_ISOLATE)
> > + return;
> > +
> > + set_pageblock_migratetype(page, MIGRATE_ISOLATE);
> > + zone->nr_pageblock_isolate++;
> > +}
> > +
> > +/* called by holding zone->lock */
> > +static void restore_pageblock_isolate(struct zone *zone, struct page *page,
> > + int migratetype)
> > +{
> > + BUG_ON(page_zone(page) != zone);
>
> ditto
>
> > + if (WARN_ON(get_pageblock_migratetype(page) != MIGRATE_ISOLATE))
> > + return;
> > +
> > + BUG_ON(zone->nr_pageblock_isolate <= 0);
> > + set_pageblock_migratetype(page, migratetype);
> > + zone->nr_pageblock_isolate--;
> > +}
>
> From: Andrew Morton <akpm@...ux-foundation.org>
> Subject: memory-hotplug-fix-kswapd-looping-forever-problem-fix
>
> simplify nr_zone_isolate_freepages(), rework zone_watermark_ok_safe() comment, simplify set_pageblock_isolate() and restore_pageblock_isolate().
>
> Cc: Aaditya Kumar <aaditya.kumar.30@...il.com>
> Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> Cc: Mel Gorman <mgorman@...e.de>
> Cc: Michal Hocko <mhocko@...e.cz>
> Cc: Minchan Kim <minchan@...nel.org>
> Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Acked-by: Minchan Kim <minchan@...nel.org>
Looks better.
Thanks, Andrew!
--
Kind regards,
Minchan Kim
--
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