linux-kernel - Re: [PATCH v2] mm/slub: Remove repeated action in calculate

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Date:   Sun, 17 Apr 2022 15:17:07 +0900
From:   Wonhyuk Yang <vvghjk1234@...il.com>
To:     Hyeonggon Yoo <42.hyeyoo@...il.com>
Cc:     Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Ohhoon Kwon <ohkwon1043@...il.com>,
        JaeSang Yoo <jsyoo5b@...il.com>,
        Jiyoup Kim <lakroforce@...il.com>,
        Donghyeok Kim <dthex5d@...il.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mm/slub: Remove repeated action in calculate_order()

On Sun, Apr 17, 2022 at 10:43 AM Hyeonggon Yoo <42.hyeyoo@...il.com> wrote:
>
> On Sat, Apr 16, 2022 at 04:40:59PM +0900, Wonhyuk Yang wrote:
> > To calculate order, calc_slab_order() is called repeatly changing the
> > fract_leftover. Thus, the branch which is not dependent on
> > fract_leftover is executed repeatly. So make it run only once.
> >
> > Plus, when min_object reached to 0, we set fract_leftover to 1. In
>
> Maybe you mean when min_object reached 1.

Yes, That comment need to be updated...

>
> > this case, we can calculate order by max(slub_min_order,
> > get_order(size)) instead of calling calc_slab_order().
> >
> > No functional impact expected.
> > Signed-off-by: Wonhyuk Yang <vvghjk1234@...il.com>
> > ---
> > V1 -> V2: Fix typo miss in a commit message
> >
> >  mm/slub.c | 18 +++++++-----------
> >  1 file changed, 7 insertions(+), 11 deletions(-)
> >
> > diff --git a/mm/slub.c b/mm/slub.c
> > index ed5c2c03a47a..e7a394d7b75a 100644
> > --- a/mm/slub.c
> > +++ b/mm/slub.c
> > @@ -3795,9 +3795,6 @@ static inline unsigned int calc_slab_order(unsigned int size,
> >       unsigned int min_order = slub_min_order;
> >       unsigned int order;
> >
> > -     if (order_objects(min_order, size) > MAX_OBJS_PER_PAGE)
> > -             return get_order(size * MAX_OBJS_PER_PAGE) - 1;
> > -
> >       for (order = max(min_order, (unsigned int)get_order(min_objects * size));
> >                       order <= max_order; order++) {
> >
> > @@ -3820,6 +3817,11 @@ static inline int calculate_order(unsigned int size)
> >       unsigned int max_objects;
> >       unsigned int nr_cpus;
> >
> > +     if (unlikely(order_objects(slub_min_order, size) > MAX_OBJS_PER_PAGE)) {
> > +             order = get_order(size * MAX_OBJS_PER_PAGE) - 1;
> > +             goto out;
> > +     }
> > +
> >       /*
> >        * Attempt to find best configuration for a slab. This
> >        * works by first attempting to generate a layout with
> > @@ -3865,14 +3867,8 @@ static inline int calculate_order(unsigned int size)
> >        * We were unable to place multiple objects in a slab. Now
> >        * lets see if we can place a single object there.
> >        */
> > -     order = calc_slab_order(size, 1, slub_max_order, 1);
> > -     if (order <= slub_max_order)
> > -             return order;
> > -
> > -     /*
> > -      * Doh this slab cannot be placed using slub_max_order.
> > -      */
> > -     order = calc_slab_order(size, 1, MAX_ORDER, 1);
> > +     order = max_t(unsigned int, slub_min_order, (unsigned int)get_order(size));
> > +out:
>
> You don't need to cast value of get_order(size). max_t() does cast both operands.

That's a good point, I will delete it.

>
> >       if (order < MAX_ORDER)
> >               return order;
> >       return -ENOSYS;
>
> For the correctness of the patch, I don't see any problem about the
> code.
>
> But to be honest I'm a bit skeptical about saving some cycles in
> calculating slab order. It's done only when creating caches (usually in boot
> process).
>