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

[KOSAKI Motohiro]

Hi

sorry for the delayed reply. I've got stucked in Larry's serious bug report awhile.

> 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.

Hm, probably I have to update my patch description.
if we use pavevec_free(), batch size is PAGEVEC_SIZE(=14).
then, order-9 lumpy reclaim makes 37 times pagevec_free(). it makes lots
temporary uncontenious memory block and the chance of stealing it from
order-0 allocator task.

This patch free all reclaimed pages at once to buddy.


> > 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.

Thank you.

> > 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.

Sure. I have to merge common block. thanks.


> > +
> >  /**
> >   * 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?

OK.

> 
> >  					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?

Honestly I didn't it yet. I only tested changing locking scheme didn't cause
reclaim throughput under light VM pressure. Probably I have to contact 
Andy and test his original problem workload.

btw, if you have good high order allocation workload, can you please tell me it?



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