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SubjectRe: [RFC][PATCH 4/4] vmscan: vmscan don't use pcp list
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@jp.fujitsu.com>
> ---
> 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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