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

[Muchun Song]

On Fri, Dec 4, 2020 at 6:28 PM Vlastimil Babka <vbabka@xxxxxxx> wrote:
>
> On 12/4/20 5:03 AM, Muchun Song wrote:
> > On Fri, Dec 4, 2020 at 1:37 AM Vlastimil Babka <vbabka@xxxxxxx> 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@xxxxxxxxxxxxx>
> >> > ---
> >> >  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.
>
> OK, I see.
>
> >> > +     if (max_order < MAX_ORDER && order < MAX_ORDER - 1) {
>
> Yes, this makes sense, as in your other patch we shouldn't check the buddy when
> order == MAX_ORDER - 1 already.
>
> >> >               /* 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;
> >> >               max_order++;
>
> OK I see now what you want to do here. the "if" may be true if we already
> entered the function with order > pageblock_order.
> I think we could just simplfy the "if" and "max_order++" above to:
>
> max_order = order + 2
>
> which starts to get a bit ugly, so why not change max_order to be -1 (compared
> to now) in the whole function:
>
> max_order = min_t(unsigned int, MAX_ORDER - 1, pageblock_order);
> ...
> continue_merging:
>         while (order < max_order) {
> ...
> if (order < MAX_ORDER - 1) {
> // it's redundant to keep checking max_order < MAX_ORDER - 1 here after your
> change, right?
> ...
>
> max_order = order + 1; // less weird than "+ 2"
>
> Off by one errors, here we go!

Great! Good suggestions. Thanks.

>
> >> 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
> >
>


-- 
Yours,
Muchun