Re: [RFC][PATCH 4/4] vmscan: vmscan don't use pcp list

[Mel Gorman]

On Fri, Nov 27, 2009 at 09:23:57AM +0900, KOSAKI Motohiro wrote:
> 
> note: Last year,  Andy Whitcroft reported pcp prevent to make contenious
> high order page when lumpy reclaim is running.

I don't remember the specifics of the discussion but I know that when
that patch series was being prototyped, it was because order-0
allocations were racing with lumpy reclaimers. A lumpy reclaim might
free up an order-9 page say but while it was freeing, an order-0 page
would be allocated from the middle. It wasn't the PCP lists as such that
were a problem once they were getting drained as part of a high-order
allocation attempt. It would be just as bad if the order-0 page was
taken from the buddy lists.

> He posted "capture pages freed during direct reclaim for allocation by the reclaimer"
> patch series, but Christoph mentioned simple bypass pcp instead.
> I made it. I'd hear Christoph and Mel's mention.
> 
> ==========================
> Currently vmscan free unused pages by __pagevec_free().  It mean free pages one by one
> and use pcp. it makes two suboptimal result.
> 
>  - The another task can steal the freed page in pcp easily. it decrease
>    lumpy reclaim worth.
>  - To pollute pcp cache, vmscan freed pages might kick out cache hot
>    pages from pcp.
> 

The latter point is interesting.

> This patch make new free_pages_bulk() function and vmscan use it.
> 
> Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
> ---
>  include/linux/gfp.h |    2 +
>  mm/page_alloc.c     |   56 +++++++++++++++++++++++++++++++++++++++++++++++++++
>  mm/vmscan.c         |   23 +++++++++++----------
>  3 files changed, 70 insertions(+), 11 deletions(-)
> 
> diff --git a/include/linux/gfp.h b/include/linux/gfp.h
> index f53e9b8..403584d 100644
> --- a/include/linux/gfp.h
> +++ b/include/linux/gfp.h
> @@ -330,6 +330,8 @@ extern void free_hot_page(struct page *page);
>  #define __free_page(page) __free_pages((page), 0)
>  #define free_page(addr) free_pages((addr),0)
>  
> +void free_pages_bulk(struct zone *zone, int count, struct list_head *list);
> +
>  void page_alloc_init(void);
>  void drain_zone_pages(struct zone *zone, struct per_cpu_pages *pcp);
>  void drain_all_pages(void);
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 11ae66e..f77f8a8 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2037,6 +2037,62 @@ void free_pages(unsigned long addr, unsigned int order)
>  
>  EXPORT_SYMBOL(free_pages);
>  
> +/*
> + * Frees a number of pages from the list
> + * Assumes all pages on list are in same zone and order==0.
> + * count is the number of pages to free.
> + *
> + * This is similar to __pagevec_free(), but receive list instead pagevec.
> + * and this don't use pcp cache. it is good characteristics for vmscan.
> + */
> +void free_pages_bulk(struct zone *zone, int count, struct list_head *list)
> +{
> +	unsigned long flags;
> +	struct page *page;
> +	struct page *page2;
> +
> +	list_for_each_entry_safe(page, page2, list, lru) {
> +		int wasMlocked = __TestClearPageMlocked(page);
> +
> +		kmemcheck_free_shadow(page, 0);
> +
> +		if (PageAnon(page))
> +			page->mapping = NULL;
> +		if (free_pages_check(page)) {
> +			/* orphan this page. */
> +			list_del(&page->lru);
> +			continue;
> +		}
> +		if (!PageHighMem(page)) {
> +			debug_check_no_locks_freed(page_address(page),
> +						   PAGE_SIZE);
> +			debug_check_no_obj_freed(page_address(page), PAGE_SIZE);
> +		}
> +		arch_free_page(page, 0);
> +		kernel_map_pages(page, 1, 0);
> +
> +		local_irq_save(flags);
> +		if (unlikely(wasMlocked))
> +			free_page_mlock(page);
> +		local_irq_restore(flags);
> +	}
> +
> +	spin_lock_irqsave(&zone->lock, flags);
> +	__count_vm_events(PGFREE, count);
> +	zone_clear_flag(zone, ZONE_ALL_UNRECLAIMABLE);
> +	zone->pages_scanned = 0;
> +
> +	__mod_zone_page_state(zone, NR_FREE_PAGES, count);
> +
> +	list_for_each_entry_safe(page, page2, list, lru) {
> +		/* have to delete it as __free_one_page list manipulates */
> +		list_del(&page->lru);
> +		trace_mm_page_free_direct(page, 0);
> +		__free_one_page(page, zone, 0, page_private(page));
> +	}
> +	spin_unlock_irqrestore(&zone->lock, flags);
> +}

It would be preferable that the bulk free code would use as much of the
existing free logic in the page allocator as possible. This is making a
lot of checks that are done elsewhere. As this is an RFC, it's not
critical but worth bearing in mind.

> +
>  /**
>   * alloc_pages_exact - allocate an exact number physically-contiguous pages.
>   * @size: the number of bytes to allocate
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 56faefb..00156f2 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -598,18 +598,17 @@ redo:
>   * shrink_page_list() returns the number of reclaimed pages
>   */
>  static unsigned long shrink_page_list(struct list_head *page_list,
> +				      struct list_head *freed_pages_list,
>  					struct scan_control *sc,

Should the freed_pages_list be part of scan_control?

>  					enum pageout_io sync_writeback)
>  {
>  	LIST_HEAD(ret_pages);
> -	struct pagevec freed_pvec;
>  	int pgactivate = 0;
>  	unsigned long nr_reclaimed = 0;
>  	unsigned long vm_flags;
>  
>  	cond_resched();
>  
> -	pagevec_init(&freed_pvec, 1);
>  	while (!list_empty(page_list)) {
>  		struct address_space *mapping;
>  		struct page *page;
> @@ -785,10 +784,7 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>  		__clear_page_locked(page);
>  free_it:
>  		nr_reclaimed++;
> -		if (!pagevec_add(&freed_pvec, page)) {
> -			__pagevec_free(&freed_pvec);
> -			pagevec_reinit(&freed_pvec);
> -		}
> +		list_add(&page->lru, freed_pages_list);
>  		continue;
>  
>  cull_mlocked:
> @@ -812,8 +808,6 @@ keep:
>  		VM_BUG_ON(PageLRU(page) || PageUnevictable(page));
>  	}
>  	list_splice(&ret_pages, page_list);
> -	if (pagevec_count(&freed_pvec))
> -		__pagevec_free(&freed_pvec);
>  	count_vm_events(PGACTIVATE, pgactivate);
>  	return nr_reclaimed;
>  }
> @@ -1100,6 +1094,7 @@ static unsigned long shrink_inactive_list(unsigned long nr_to_scan,
>  					  int priority, int file)
>  {
>  	LIST_HEAD(page_list);
> +	LIST_HEAD(freed_pages_list);
>  	struct pagevec pvec;
>  	unsigned long nr_scanned;
>  	unsigned long nr_reclaimed = 0;
> @@ -1174,7 +1169,8 @@ static unsigned long shrink_inactive_list(unsigned long nr_to_scan,
>  
>  	spin_unlock_irq(&zone->lru_lock);
>  
> -	nr_reclaimed = shrink_page_list(&page_list, sc, PAGEOUT_IO_ASYNC);
> +	nr_reclaimed = shrink_page_list(&page_list, &freed_pages_list, sc,
> +					PAGEOUT_IO_ASYNC);
>  
>  	/*
>  	 * If we are direct reclaiming for contiguous pages and we do
> @@ -1192,10 +1188,15 @@ static unsigned long shrink_inactive_list(unsigned long nr_to_scan,
>  		nr_active = clear_active_flags(&page_list, count);
>  		count_vm_events(PGDEACTIVATE, nr_active);
>  
> -		nr_reclaimed += shrink_page_list(&page_list, sc,
> -						 PAGEOUT_IO_SYNC);
> +		nr_reclaimed += shrink_page_list(&page_list, &freed_pages_list,
> +						 sc, PAGEOUT_IO_SYNC);
>  	}
>  
> +	/*
> +	 * Free unused pages.
> +	 */
> +	free_pages_bulk(zone, nr_reclaimed, &freed_pages_list);
> +
>  	local_irq_disable();
>  	if (current_is_kswapd())
>  		__count_vm_events(KSWAPD_STEAL, nr_reclaimed);

This patch does not stand-alone so it's not easy to test. I'll think about
the idea more although I do see how it might help slightly in the same way
capture-reclaim did by closing the race window with other allocators.

I'm curious, how did you evaluate this and what problem did you
encounter that this might help?

Thanks

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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