Date:	Thu, 28 Jun 2012 16:13:48 +0900
From:	Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Minchan Kim <minchan@...nel.org>
CC:	akpm@...ux-foundation.org,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	Mel Gorman <mel@....ul.ie>,
	Aaditya Kumar <aaditya.kumar.30@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>, Mel Gorman <mgorman@...e.de>
Subject: Re: [PATCH 2/2 v2] memory-hotplug: fix kswapd looping forever problem

(2012/06/27 16:51), Minchan Kim 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@...fujitsu.com>
> Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> Cc: Aaditya Kumar <aaditya.kumar.30@...il.com>
> Cc: Mel Gorman <mgorman@...e.de>
> Signed-off-by: Minchan Kim <minchan@...nel.org>
> ---
> 
> Aaditya, coul you confirm this patch solve your problem and
> make sure nr_pageblock_isolate is zero after hotplug end?
> 
> Thanks!
> 
>   include/linux/mmzone.h |    8 ++++++++
>   mm/page_alloc.c        |   31 +++++++++++++++++++++++++++++++
>   mm/page_isolation.c    |   29 +++++++++++++++++++++++++++--
>   3 files changed, 66 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index dbc876e..6ee83b8 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -474,6 +474,14 @@ struct zone {
>   	 * rarely used fields:
>   	 */
>   	const char		*name;
> +#ifdef CONFIG_MEMORY_ISOLATION
> +	/*
> +	 * the number of MIGRATE_ISOLATE *pageblock*.
> +	 * We need this for free page counting. Look at zone_watermark_ok_safe.
> +	 * It's protected by zone->lock
> +	 */
> +	int		nr_pageblock_isolate;
> +#endif
>   } ____cacheline_internodealigned_in_smp;
>   
>   typedef enum {
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index c175fa9..b12c8ec 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -218,6 +218,11 @@ EXPORT_SYMBOL(nr_online_nodes);
>   
>   int page_group_by_mobility_disabled __read_mostly;
>   
> +/*
> + * NOTE:
> + * Don't use set_pageblock_migratetype(page, MIGRATE_ISOLATE) directly.
> + * Instead, use {un}set_pageblock_isolate.
> + */
>   void set_pageblock_migratetype(struct page *page, int migratetype)
>   {
>   	if (unlikely(page_group_by_mobility_disabled))
> @@ -1614,6 +1619,23 @@ static bool __zone_watermark_ok(struct zone *z, int order, unsigned long mark,
>   	return true;
>   }
>   
> +#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);

Here, free_pages could be < 0 ?

Thanks,
-Kame

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