Re: [PATCH v2] mm/slub: Remove repeated action in calculate_order()

[Hyeonggon Yoo <42.hyeyoo@gmail.com>]

On Sun, Apr 17, 2022 at 10:43:20AM +0900, Hyeonggon Yoo 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.
> 
> > 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@gmail.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.
> 
> >  	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).

But yeah, maybe it's worth for better maintenance of code.

So after considering my comments, feel free to add:
Reviewed-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>

Thanks!

> > -- 
> > 2.30.2
> > 
> > 
> 
> -- 
> Thanks,
> Hyeonggon

-- 
Thanks,
Hyeonggon