Re: [RFC PATCH v2 2/4] mm/__free_one_page: skip merge for order-0 page unless compaction failed

["Figo.zhang" <figo1802@gmail.com>]

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2018-03-20 1:54 GMT-07:00 Aaron Lu <aaron.lu@intel.com>:

> Running will-it-scale/page_fault1 process mode workload on a 2 sockets
> Intel Skylake server showed severe lock contention of zone->lock, as
> high as about 80%(42% on allocation path and 35% on free path) CPU
> cycles are burnt spinning. With perf, the most time consuming part inside
> that lock on free path is cache missing on page structures, mostly on
> the to-be-freed page's buddy due to merging.
>
> One way to avoid this overhead is not do any merging at all for order-0
> pages. With this approach, the lock contention for zone->lock on free
> path dropped to 1.1% but allocation side still has as high as 42% lock
> contention. In the meantime, the dropped lock contention on free side
> doesn't translate to performance increase, instead, it's consumed by
> increased lock contention of the per node lru_lock(rose from 5% to 37%)
> and the final performance slightly dropped about 1%.
>
> Though performance dropped a little, it almost eliminated zone lock
> contention on free path and it is the foundation for the next patch
> that eliminates zone lock contention for allocation path.
>
> A new document file called "struct_page_filed" is added to explain
> the newly reused field in "struct page".
>
> Suggested-by: Dave Hansen <dave.hansen@intel.com>
> Signed-off-by: Aaron Lu <aaron.lu@intel.com>
> ---
>  Documentation/vm/struct_page_field |  5 +++
>  include/linux/mm_types.h           |  1 +
>  mm/compaction.c                    | 13 +++++-
>  mm/internal.h                      | 27 ++++++++++++
>  mm/page_alloc.c                    | 89 ++++++++++++++++++++++++++++++
> +++-----
>  5 files changed, 122 insertions(+), 13 deletions(-)
>  create mode 100644 Documentation/vm/struct_page_field
>
> diff --git a/Documentation/vm/struct_page_field b/Documentation/vm/struct_
> page_field
> new file mode 100644
> index 000000000000..1ab6c19ccc7a
> --- /dev/null
> +++ b/Documentation/vm/struct_page_field
> @@ -0,0 +1,5 @@
> +buddy_merge_skipped:
> +Used to indicate this page skipped merging when added to buddy. This
> +field only makes sense if the page is in Buddy and is order zero.
> +It's a bug if any higher order pages in Buddy has this field set.
> +Shares space with index.
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index fd1af6b9591d..7edc4e102a8e 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -91,6 +91,7 @@ struct page {
>                 pgoff_t index;          /* Our offset within mapping. */
>                 void *freelist;         /* sl[aou]b first free object */
>                 /* page_deferred_list().prev    -- second tail page */
> +               bool buddy_merge_skipped; /* skipped merging when added to
> buddy */
>         };
>
>         union {
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 2c8999d027ab..fb9031fdca41 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -776,8 +776,19 @@ isolate_migratepages_block(struct compact_control
> *cc, unsigned long low_pfn,
>                  * potential isolation targets.
>                  */
>                 if (PageBuddy(page)) {
> -                       unsigned long freepage_order =
> page_order_unsafe(page);
> +                       unsigned long freepage_order;
>
> +                       /*
> +                        * If this is a merge_skipped page, do merge now
> +                        * since high-order pages are needed. zone lock
> +                        * isn't taken for the merge_skipped check so the
> +                        * check could be wrong but the worst case is we
> +                        * lose a merge opportunity.
> +                        */
> +                       if (page_merge_was_skipped(page))
> +                               try_to_merge_page(page);
> +
> +                       freepage_order = page_order_unsafe(page);
>                         /*
>                          * Without lock, we cannot be sure that what we
> got is
>                          * a valid page order. Consider only values in the
>

when the system memory is very very low and try a lot of failures and then
go into
__alloc_pages_direct_compact() to has a opportunity to do your
try_to_merge_page(), is it the best timing for here to
do order-0 migration?

diff --git a/mm/internal.h b/mm/internal.h
> index e6bd35182dae..2bfbaae2d835 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -538,4 +538,31 @@ static inline bool is_migrate_highatomic_page(struct
> page *page)
>  }
>
>  void setup_zone_pageset(struct zone *zone);
> +
> +static inline bool page_merge_was_skipped(struct page *page)
> +{
> +       return page->buddy_merge_skipped;
> +}
> +
> +void try_to_merge_page(struct page *page);
> +
> +#ifdef CONFIG_COMPACTION
> +static inline bool can_skip_merge(struct zone *zone, int order)
> +{
> +       /* Compaction has failed in this zone, we shouldn't skip merging */
> +       if (zone->compact_considered)
> +               return false;
> +
> +       /* Only consider no_merge for order 0 pages */
> +       if (order)
> +               return false;
> +
> +       return true;
> +}
> +#else /* CONFIG_COMPACTION */
> +static inline bool can_skip_merge(struct zone *zone, int order)
> +{
> +       return false;
> +}
> +#endif  /* CONFIG_COMPACTION */
>  #endif /* __MM_INTERNAL_H */
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 3cdf1e10d412..eb78014dfbde 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -730,6 +730,16 @@ static inline void clear_page_guard(struct zone
> *zone, struct page *page,
>                                 unsigned int order, int migratetype) {}
>  #endif
>
> +static inline void set_page_merge_skipped(struct page *page)
> +{
> +       page->buddy_merge_skipped = true;
> +}
> +
> +static inline void clear_page_merge_skipped(struct page *page)
> +{
> +       page->buddy_merge_skipped = false;
> +}
> +
>  static inline void set_page_order(struct page *page, unsigned int order)
>  {
>         set_page_private(page, order);
> @@ -739,6 +749,13 @@ static inline void set_page_order(struct page *page,
> unsigned int order)
>  static inline void add_to_buddy_common(struct page *page, struct zone
> *zone,
>                                         unsigned int order, int mt)
>  {
> +       /*
> +        * Always clear buddy_merge_skipped when added to buddy because
> +        * buddy_merge_skipped shares space with index and index could
> +        * be used as migratetype for PCP pages.
> +        */
> +       clear_page_merge_skipped(page);
> +
>         set_page_order(page, order);
>         zone->free_area[order].nr_free++;
>  }
> @@ -769,6 +786,7 @@ static inline void remove_from_buddy(struct page
> *page, struct zone *zone,
>         list_del(&page->lru);
>         zone->free_area[order].nr_free--;
>         rmv_page_order(page);
> +       clear_page_merge_skipped(page);
>  }
>
>  /*
> @@ -839,7 +857,7 @@ static inline int page_is_buddy(struct page *page,
> struct page *buddy,
>   * -- nyc
>   */
>
> -static inline void __free_one_page(struct page *page,
> +static inline void do_merge(struct page *page,
>                 unsigned long pfn,
>                 struct zone *zone, unsigned int order,
>                 int migratetype)
> @@ -851,16 +869,6 @@ static inline void __free_one_page(struct page *page,
>
>         max_order = min_t(unsigned int, MAX_ORDER, pageblock_order + 1);
>
> -       VM_BUG_ON(!zone_is_initialized(zone));
> -       VM_BUG_ON_PAGE(page->flags & PAGE_FLAGS_CHECK_AT_PREP, page);
> -
> -       VM_BUG_ON(migratetype == -1);
> -       if (likely(!is_migrate_isolate(migratetype)))
> -               __mod_zone_freepage_state(zone, 1 << order, migratetype);
> -
> -       VM_BUG_ON_PAGE(pfn & ((1 << order) - 1), page);
> -       VM_BUG_ON_PAGE(bad_range(zone, page), page);
> -
>  continue_merging:
>         while (order < max_order - 1) {
>                 buddy_pfn = __find_buddy_pfn(pfn, order);
> @@ -933,6 +941,61 @@ static inline void __free_one_page(struct page *page,
>         add_to_buddy_head(page, zone, order, migratetype);
>  }
>
> +void try_to_merge_page(struct page *page)
> +{
> +       unsigned long pfn, buddy_pfn, flags;
> +       struct page *buddy;
> +       struct zone *zone;
> +
> +       /*
> +        * No need to do merging if buddy is not free.
> +        * zone lock isn't taken so this could be wrong but worst case
> +        * is we lose a merge opportunity.
> +        */
> +       pfn = page_to_pfn(page);
> +       buddy_pfn = __find_buddy_pfn(pfn, 0);
> +       buddy = page + (buddy_pfn - pfn);
> +       if (!PageBuddy(buddy))
> +               return;
> +
> +       zone = page_zone(page);
> +       spin_lock_irqsave(&zone->lock, flags);
> +       /* Verify again after taking the lock */
> +       if (likely(PageBuddy(page) && page_merge_was_skipped(page) &&
> +                  PageBuddy(buddy))) {
> +               int mt = get_pageblock_migratetype(page);
> +
> +               remove_from_buddy(page, zone, 0);
> +               do_merge(page, pfn, zone, 0, mt);
> +       }
> +       spin_unlock_irqrestore(&zone->lock, flags);
> +}
> +
> +static inline void __free_one_page(struct page *page,
> +               unsigned long pfn,
> +               struct zone *zone, unsigned int order,
> +               int migratetype)
> +{
> +       VM_BUG_ON(!zone_is_initialized(zone));
> +       VM_BUG_ON_PAGE(page->flags & PAGE_FLAGS_CHECK_AT_PREP, page);
> +
> +       VM_BUG_ON(migratetype == -1);
> +       if (likely(!is_migrate_isolate(migratetype)))
> +               __mod_zone_freepage_state(zone, 1 << order, migratetype);
> +
> +       VM_BUG_ON_PAGE(pfn & ((1 << order) - 1), page);
> +       VM_BUG_ON_PAGE(bad_range(zone, page), page);
> +
> +       if (can_skip_merge(zone, order)) {
> +               add_to_buddy_head(page, zone, 0, migratetype);
> +               set_page_merge_skipped(page);
> +               return;
> +       }
> +
> +       do_merge(page, pfn, zone, order, migratetype);
> +}
> +
> +
>  /*
>   * A bad page could be due to a number of fields. Instead of multiple
> branches,
>   * try and check multiple fields with one check. The caller must do a
> detailed
> @@ -1183,8 +1246,10 @@ static void free_pcppages_bulk(struct zone *zone,
> int count,
>                          * can be offset by reduced memory latency later.
> To
>                          * avoid excessive prefetching due to large count,
> only
>                          * prefetch buddy for the last pcp->batch nr of
> pages.
> +                        *
> +                        * If merge can be skipped, no need to prefetch
> buddy.
>                          */
> -                       if (count > pcp->batch)
> +                       if (can_skip_merge(zone, 0) || count > pcp->batch)
>                                 continue;
>                         pfn = page_to_pfn(page);
>                         buddy_pfn = __find_buddy_pfn(pfn, 0);
> --
> 2.14.3
>
>

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