linux-kernel - Re: [PATCH 4/4] mm/slub: refactor calculate_order() and calc_slab

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Date:   Sat, 16 Sep 2023 09:28:49 +0800
From:   Baoquan He <bhe@...hat.com>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     David Rientjes <rientjes@...gle.com>,
        Christoph Lameter <cl@...ux.com>,
        Hyeonggon Yoo <42.hyeyoo@...il.com>,
        Jay Patel <jaypatel@...ux.ibm.com>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Pekka Enberg <penberg@...nel.org>,
        Joonsoo Kim <iamjoonsoo.kim@....com>, linux-mm@...ck.org,
        patches@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/4] mm/slub: refactor calculate_order() and
 calc_slab_order()

On 09/08/23 at 04:53pm, Vlastimil Babka wrote:
> After the previous cleanups, we can now move some code from
> calc_slab_order() to calculate_order() so it's executed just once, and
> do some more cleanups.
> 
> - move the min_order and MAX_OBJS_PER_PAGE evaluation to
>   calc_slab_order().
> 
> - change calc_slab_order() parameter min_objects to min_order
> 
> Also make MAX_OBJS_PER_PAGE check more robust by considering also
> min_objects in addition to slub_min_order. Otherwise this is not a
> functional change.
> 
> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
> ---
>  mm/slub.c | 19 +++++++++----------
>  1 file changed, 9 insertions(+), 10 deletions(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index f04eb029d85a..1c91f72c7239 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -4110,17 +4110,12 @@ static unsigned int slub_min_objects;
>   * the smallest order which will fit the object.
>   */
>  static inline unsigned int calc_slab_order(unsigned int size,
> -		unsigned int min_objects, unsigned int max_order,
> +		unsigned int min_order, unsigned int max_order,
>  		unsigned int fract_leftover)
>  {
> -	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++) {
> +	for (order = min_order; order <= max_order; order++) {
>  
>  		unsigned int slab_size = (unsigned int)PAGE_SIZE << order;
>  		unsigned int rem;
> @@ -4139,7 +4134,7 @@ static inline int calculate_order(unsigned int size)
>  	unsigned int order;
>  	unsigned int min_objects;
>  	unsigned int max_objects;
> -	unsigned int nr_cpus;
> +	unsigned int min_order;
>  
>  	min_objects = slub_min_objects;
>  	if (!min_objects) {
> @@ -4152,7 +4147,7 @@ static inline int calculate_order(unsigned int size)
>  		 * order on systems that appear larger than they are, and too
>  		 * low order on systems that appear smaller than they are.
>  		 */
> -		nr_cpus = num_present_cpus();
> +		unsigned int nr_cpus = num_present_cpus();
>  		if (nr_cpus <= 1)
>  			nr_cpus = nr_cpu_ids;
>  		min_objects = 4 * (fls(nr_cpus) + 1);

A minor concern, should we change 'min_objects' to be a local static
to avoid the "if (!min_objects) {" code block every time?  It's deducing
the value from nr_cpus, we may not need do the calculation each time.

> @@ -4160,6 +4155,10 @@ static inline int calculate_order(unsigned int size)
>  	max_objects = order_objects(slub_max_order, size);
>  	min_objects = min(min_objects, max_objects);
>  
> +	min_order = max(slub_min_order, (unsigned int)get_order(min_objects * size));
> +	if (order_objects(min_order, size) > MAX_OBJS_PER_PAGE)
> +		return get_order(size * MAX_OBJS_PER_PAGE) - 1;
> +
>  	/*
>  	 * Attempt to find best configuration for a slab. This works by first
>  	 * attempting to generate a layout with the best possible configuration and
> @@ -4176,7 +4175,7 @@ static inline int calculate_order(unsigned int size)
>  	 * long as at least single object fits within slub_max_order.
>  	 */
>  	for (unsigned int fraction = 16; fraction > 1; fraction /= 2) {
> -		order = calc_slab_order(size, min_objects, slub_max_order,
> +		order = calc_slab_order(size, min_order, slub_max_order,
>  					fraction);
>  		if (order <= slub_max_order)
>  			return order;
> -- 
> 2.42.0
>