Date:	Sat, 30 Apr 2011 00:47:03 +0900
From:	Minchan Kim <minchan.kim@...il.com>
To:	Mel Gorman <mgorman@...e.de>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-mm <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Christoph Lameter <cl@...ux.com>,
	Johannes Weiner <jweiner@...hat.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Rik van Riel <riel@...hat.com>,
	Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: [RFC 6/8] In order putback lru core

On Thu, Apr 28, 2011 at 8:06 PM, Mel Gorman <mgorman@...e.de> wrote:
> On Wed, Apr 27, 2011 at 01:25:23AM +0900, Minchan Kim wrote:
>> This patch defines new APIs to putback the page into previous position of LRU.
>> The idea is simple.
>>
>> When we try to putback the page into lru list and if friends(prev, next) of the pages
>> still is nearest neighbor, we can insert isolated page into prev's next instead of
>> head of LRU list. So it keeps LRU history without losing the LRU information.
>>
>> Before :
>>       LRU POV : H - P1 - P2 - P3 - P4 -T
>>
>> Isolate P3 :
>>       LRU POV : H - P1 - P2 - P4 - T
>>
>> Putback P3 :
>>       if (P2->next == P4)
>>               putback(P3, P2);
>>       So,
>>       LRU POV : H - P1 - P2 - P3 - P4 -T
>>
>> For implement, we defines new structure pages_lru which remebers
>> both lru friend pages of isolated one and handling functions.
>>
>> But this approach has a problem on contiguous pages.
>> In this case, my idea can not work since friend pages are isolated, too.
>> It means prev_page->next == next_page always is false and both pages are not
>> LRU any more at that time. It's pointed out by Rik at LSF/MM summit.
>> So for solving the problem, I can change the idea.
>> I think we don't need both friend(prev, next) pages relation but
>> just consider either prev or next page that it is still same LRU.
>> Worset case in this approach, prev or next page is free and allocate new
>> so it's in head of LRU and our isolated page is located on next of head.
>> But it's almost same situation with current problem. So it doesn't make worse
>> than now and it would be rare. But in this version, I implement based on idea
>> discussed at LSF/MM. If my new idea makes sense, I will change it.
>>
>> Any comment?
>>
>> Cc: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
>> Cc: Mel Gorman <mgorman@...e.de>
>> Cc: Rik van Riel <riel@...hat.com>
>> Cc: Andrea Arcangeli <aarcange@...hat.com>
>> Signed-off-by: Minchan Kim <minchan.kim@...il.com>
>> ---
>>  include/linux/migrate.h  |    2 +
>>  include/linux/mm_types.h |    7 ++++
>>  include/linux/swap.h     |    4 ++-
>>  mm/compaction.c          |    3 +-
>>  mm/internal.h            |    2 +
>>  mm/memcontrol.c          |    2 +-
>>  mm/migrate.c             |   36 +++++++++++++++++++++
>>  mm/swap.c                |    2 +-
>>  mm/vmscan.c              |   79 +++++++++++++++++++++++++++++++++++++++++++--
>>  9 files changed, 129 insertions(+), 8 deletions(-)
>>
>> diff --git a/include/linux/migrate.h b/include/linux/migrate.h
>> index e39aeec..3aa5ab6 100644
>> --- a/include/linux/migrate.h
>> +++ b/include/linux/migrate.h
>> @@ -9,6 +9,7 @@ typedef struct page *new_page_t(struct page *, unsigned long private, int **);
>>  #ifdef CONFIG_MIGRATION
>>  #define PAGE_MIGRATION 1
>>
>> +extern void putback_pages_lru(struct list_head *l);
>>  extern void putback_lru_pages(struct list_head *l);
>>  extern int migrate_page(struct address_space *,
>>                       struct page *, struct page *);
>> @@ -33,6 +34,7 @@ extern int migrate_huge_page_move_mapping(struct address_space *mapping,
>>  #else
>>  #define PAGE_MIGRATION 0
>>
>> +static inline void putback_pages_lru(struct list_head *l) {}
>>  static inline void putback_lru_pages(struct list_head *l) {}
>>  static inline int migrate_pages(struct list_head *l, new_page_t x,
>>               unsigned long private, bool offlining,
>> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
>> index ca01ab2..35e80fb 100644
>> --- a/include/linux/mm_types.h
>> +++ b/include/linux/mm_types.h
>> @@ -102,6 +102,13 @@ struct page {
>>  #endif
>>  };
>>
>> +/* This structure is used for keeping LRU ordering of isolated page */
>> +struct pages_lru {
>> +        struct page *page;      /* isolated page */
>> +        struct page *prev_page; /* previous page of isolate page as LRU order */
>> +        struct page *next_page; /* next page of isolate page as LRU order */
>> +        struct list_head lru;
>> +};
>>  /*
>
> So this thing has to be allocated from somewhere. We can't put it
> on the stack as we're already in danger there so we must be using
> kmalloc. In the reclaim paths, this should be avoided obviously.
> For compaction, we might hurt the compaction success rates if pages
> are pinned with control structures. It's something to be wary of.

Actually, 32 page unit of migration in compaction should be not a big problem.
Maybe, just one page is enough to keep pages_lru arrays but I admit
dynamic allocation in reclaim patch and pinning the page in compaction
path isn't good.
So I have thought pagevec-like approach which is static allocated.
But the approach should be assume the migration unit should be small
like SWAP_CLUSTER_MAX It's true now but actually I don't want to
depends on the size.
But for free the size limitation, we have to allocate new page dynamically,

Hmm, any ideas?
Personally, I don't have any idea other than pagevec-like approach of 32 pages.

>
> At LSF/MM, I stated a preference for swapping the source and
> destination pages in the LRU. This unfortunately means that the LRU

I can't understand your point. swapping source and destination?
Could you elaborate on it?

> now contains a page in the process of being migrated to and the backout
> paths for migration failure become a lot more complex. Reclaim should
> be ok as it'll should fail to lock the page and recycle it in the list.
> This avoids allocations but I freely admit that I'm not in the position
> to implement such a thing right now :(
>
>>   * A region containing a mapping of a non-memory backed file under NOMMU
>>   * conditions.  These are held in a global tree and are pinned by the VMAs that
>> diff --git a/include/linux/swap.h b/include/linux/swap.h
>> index baef4ad..4ad0a0c 100644
>> --- a/include/linux/swap.h
>> +++ b/include/linux/swap.h
>> @@ -227,6 +227,8 @@ extern void rotate_reclaimable_page(struct page *page);
>>  extern void deactivate_page(struct page *page);
>>  extern void swap_setup(void);
>>
>> +extern void update_page_reclaim_stat(struct zone *zone, struct page *page,
>> +                                    int file, int rotated);
>>  extern void add_page_to_unevictable_list(struct page *page);
>>
>>  /**
>> @@ -260,7 +262,7 @@ extern unsigned long mem_cgroup_shrink_node_zone(struct mem_cgroup *mem,
>>                                               struct zone *zone,
>>                                               unsigned long *nr_scanned);
>>  extern int __isolate_lru_page(struct page *page, int mode, int file,
>> -                             int not_dirty, int not_mapped);
>> +             int not_dirty, int not_mapped, struct pages_lru *pages_lru);
>>  extern unsigned long shrink_all_memory(unsigned long nr_pages);
>>  extern int vm_swappiness;
>>  extern int remove_mapping(struct address_space *mapping, struct page *page);
>> diff --git a/mm/compaction.c b/mm/compaction.c
>> index 653b02b..c453000 100644
>> --- a/mm/compaction.c
>> +++ b/mm/compaction.c
>> @@ -335,7 +335,8 @@ static unsigned long isolate_migratepages(struct zone *zone,
>>               }
>>
>>               /* Try isolate the page */
>> -             if (__isolate_lru_page(page, ISOLATE_BOTH, 0, !cc->sync, 0) != 0)
>> +             if (__isolate_lru_page(page, ISOLATE_BOTH, 0,
>> +                                     !cc->sync, 0, NULL) != 0)
>>                       continue;
>>
>>               VM_BUG_ON(PageTransCompound(page));
>> diff --git a/mm/internal.h b/mm/internal.h
>> index d071d38..3c8182c 100644
>> --- a/mm/internal.h
>> +++ b/mm/internal.h
>> @@ -43,6 +43,8 @@ extern unsigned long highest_memmap_pfn;
>>   * in mm/vmscan.c:
>>   */
>>  extern int isolate_lru_page(struct page *page);
>> +extern bool keep_lru_order(struct pages_lru *pages_lru);
>> +extern void putback_page_to_lru(struct page *page, struct list_head *head);
>>  extern void putback_lru_page(struct page *page);
>>
>>  /*
>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>> index 471e7fd..92a9046 100644
>> --- a/mm/memcontrol.c
>> +++ b/mm/memcontrol.c
>> @@ -1193,7 +1193,7 @@ unsigned long mem_cgroup_isolate_pages(unsigned long nr_to_scan,
>>                       continue;
>>
>>               scan++;
>> -             ret = __isolate_lru_page(page, mode, file, 0, 0);
>> +             ret = __isolate_lru_page(page, mode, file, 0, 0, NULL);
>>               switch (ret) {
>>               case 0:
>>                       list_move(&page->lru, dst);
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index 819d233..9cfb63b 100644
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -85,6 +85,42 @@ void putback_lru_pages(struct list_head *l)
>>  }
>>
>>  /*
>> + * This function is almost same iwth putback_lru_pages.
>> + * The difference is that function receives struct pages_lru list
>> + * and if possible, we add pages into original position of LRU
>> + * instead of LRU's head.
>> + */
>> +void putback_pages_lru(struct list_head *l)
>> +{
>> +        struct pages_lru *isolated_page;
>> +        struct pages_lru *isolated_page2;
>> +        struct page *page;
>> +
>> +        list_for_each_entry_safe(isolated_page, isolated_page2, l, lru) {
>> +                struct zone *zone;
>> +                page = isolated_page->page;
>> +                list_del(&isolated_page->lru);
>> +
>> +                dec_zone_page_state(page, NR_ISOLATED_ANON +
>> +                                page_is_file_cache(page));
>> +
>> +                zone = page_zone(page);
>> +                spin_lock_irq(&zone->lru_lock);
>> +                if (keep_lru_order(isolated_page)) {
>> +                        putback_page_to_lru(page, &isolated_page->prev_page->lru);
>> +                        spin_unlock_irq(&zone->lru_lock);
>> +                }
>> +                else {
>> +                        spin_unlock_irq(&zone->lru_lock);
>> +                        putback_lru_page(page);
>> +                }
>> +
>> +                kfree(isolated_page);
>> +        }
>> +}
>
> I think we also need counters at least at discussion stage to see
> how successful this is.

Actually, I had some numbers but don't published it.
(4core, 1G DRAM and THP always, big stress(qemu, compile, web
browsing, eclipse and other programs fork, successful rate is about
50%)
That's because I have another idea to solve contiguous PFN problem.
Please, see the description and reply of Rik's question on this thread.
I guess the approach could make a high successful rate if there isn't
concurrent direct compaction processes.

>
> For example, early in the system there is a casual relationship
> between the age of a page and its location in physical memory. The
> buddy allocator gives pages back in PFN order where possible and
> there is a loose relationship between when pages get allocated and
> when they get reclaimed. As compaction is a linear scanner, there
> is a likelihood (that is highly variable) that physically contiguous
> pages will have similar positions in the LRU. They'll be isolated at
> the same time meaning they also won't be put back in order.

Indeed! but I hope my second idea would solve the problem.
What do you think about it?
>
> This might cease to matter when the system is running for some time but
> it's a concern.

Yes. It depends on aging of workloads.
I think it's not easy to get a meaningful number to justify all. :(

-- 
Kind regards,
Minchan Kim
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