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

[Muchun Song <songmuchun@bytedance.com>]

On Fri, Dec 4, 2020 at 1:37 AM Vlastimil Babka <vbabka@suse.cz> wrote:
>
> On 12/2/20 1:18 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
>
> But we have to do that. It's not the same page, it's the merged page and the new
> buddy is a different pageblock and we need to check if they have compatible
> migratetypes and can merge, or we have to bail out. So the patch is wrong.
>
> > repeatedly especially when MAX_ORDER is much larger than pageblock_order.
>
> Do we have such architectures/configurations anyway?
>
> > Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> > ---
> >  mm/page_alloc.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 141f12e5142c..959541234e1d 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -1041,7 +1041,7 @@ static inline void __free_one_page(struct page *page,
> >               pfn = combined_pfn;
> >               order++;
> >       }
> > -     if (max_order < MAX_ORDER) {

If we free a page with order == MAX_ORDER - 1, it has no buddy.
The following pageblock operation is also pointless.

> > +     if (max_order < MAX_ORDER && 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
> > @@ -1062,6 +1062,8 @@ static inline void __free_one_page(struct page *page,
> >                                               is_migrate_isolate(buddy_mt)))
> >                               goto done_merging;
> >               }
> > +             if (unlikely(order != max_order - 1))
> > +                     max_order = order + 1;
>
> Or maybe I just don't understand what this is doing. When is the new 'if' even
> true? We just bailed out of "while (order < max_order - 1)" after the last
> "order++", which means it should hold that "order == max_order - 1")?

No, I do not agree. The MAX_ORDER may be greater than 11.

# git grep "CONFIG_FORCE_MAX_ZONEORDER"
# arch/arm/configs/imx_v6_v7_defconfig:CONFIG_FORCE_MAX_ZONEORDER=14
# arch/powerpc/configs/85xx/ge_imp3a_defconfig:CONFIG_FORCE_MAX_ZONEORDER=17
# arch/powerpc/configs/fsl-emb-nonhw.config:CONFIG_FORCE_MAX_ZONEORDER=13

Have you seen it? On some architecture, the MAX_ORDER
can be 17. When we free a page with an order 16. Without this
patch, the max_order should be increased one by one from 10 to
17.

Thanks.


> Your description sounds like you want to increase max_order to MAX_ORDER in one
> step, which as I explained would be wrong. But the implementation looks actually
> like a no-op.
>
> >               max_order++;
> >               goto continue_merging;
> >       }
> >
>


--
Yours,
Muchun