Re: [External] Re: [PATCH v2] mm/page_alloc: speeding up the iteration of max_order

[Muchun Song <songmuchun@bytedance.com>]

On Fri, Dec 4, 2020 at 11:28 PM Vlastimil Babka <vbabka@suse.cz> wrote:
>
> On 12/4/20 1:56 PM, Muchun Song wrote:
> > When we free a page whose order is very close to MAX_ORDER and greater
> > than pageblock_order, it wastes some CPU cycles to increase max_order
> > to MAX_ORDER one by one and check the pageblock migratetype of that page
> > repeatedly especially when MAX_ORDER is much larger than pageblock_order.
>
> I would add:
>
> We also should not be checking migratetype of buddy when "order == MAX_ORDER -
> 1" as the buddy pfn may be invalid, so adjust the condition. With the new check,
> we don't need the max_order check anymore, so we replace it.
>
> Also adjust max_order initialization so that it's lower by one than previously,
> which makes the code hopefully more clear.

Got it. Thanks.

>
> > Signed-off-by: Muchun Song <songmuchun@bytedance.com>
>
> Fixes: d9dddbf55667 ("mm/page_alloc: prevent merging between isolated and other
> pageblocks")
> Acked-by: Vlastimil Babka <vbabka@suse.cz>
> Thanks!
>
> > ---
> > Changes in v2:
> >  - Rework the code suggested by Vlastimil. Thanks.
> >
> >  mm/page_alloc.c | 8 ++++----
> >  1 file changed, 4 insertions(+), 4 deletions(-)
> >
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index f91df593bf71..56e603eea1dd 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -1002,7 +1002,7 @@ static inline void __free_one_page(struct page *page,
> >       struct page *buddy;
> >       bool to_tail;
> >
> > -     max_order = min_t(unsigned int, MAX_ORDER, pageblock_order + 1);
> > +     max_order = min_t(unsigned int, MAX_ORDER - 1, pageblock_order);
> >
> >       VM_BUG_ON(!zone_is_initialized(zone));
> >       VM_BUG_ON_PAGE(page->flags & PAGE_FLAGS_CHECK_AT_PREP, page);
> > @@ -1015,7 +1015,7 @@ static inline void __free_one_page(struct page *page,
> >       VM_BUG_ON_PAGE(bad_range(zone, page), page);
> >
> >  continue_merging:
> > -     while (order < max_order - 1) {
> > +     while (order < max_order) {
> >               if (compaction_capture(capc, page, order, migratetype)) {
> >                       __mod_zone_freepage_state(zone, -(1 << order),
> >                                                               migratetype);
> > @@ -1041,7 +1041,7 @@ static inline void __free_one_page(struct page *page,
> >               pfn = combined_pfn;
> >               order++;
> >       }
> > -     if (max_order < MAX_ORDER) {
> > +     if (order < MAX_ORDER - 1) {
> >               /* If we are here, it means order is >= pageblock_order.
> >                * We want to prevent merge between freepages on isolate
> >                * pageblock and normal pageblock. Without this, pageblock
> > @@ -1062,7 +1062,7 @@ static inline void __free_one_page(struct page *page,
> >                                               is_migrate_isolate(buddy_mt)))
> >                               goto done_merging;
> >               }
> > -             max_order++;
> > +             max_order = order + 1;
> >               goto continue_merging;
> >       }
> >
> >
>


-- 
Yours,
Muchun