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

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Message-ID: <210044a1-1f07-b579-bdf5-3f9ac8fbdc8c@suse.cz>
Date:   Fri, 22 Sep 2023 08:55:27 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Feng Tang <feng.tang@...el.com>
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" <linux-mm@...ck.org>,
        "patches@...ts.linux.dev" <patches@...ts.linux.dev>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/4] mm/slub: refactor calculate_order() and
 calc_slab_order()

On 9/20/23 15:36, Feng Tang wrote:
> On Fri, Sep 08, 2023 at 10:53:07PM +0800, 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().
> 
> Nit: here is to 'move ... to calculate_order()'?

Oops, right, fixed.

> I tried this patch series with normal boot on a desktop and one 2
> socket server: patch 2/4 doesn't change order of any slab, and patch
> 3/4 does make the slab order of big objects more consistent.
> 
> Thanks for making the code much cleaner! And for the whole series, 
> 
> Reviewed-by: Feng Tang <feng.tang@...el.com>

Thanks! Applied.

> 
>> - 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);
>> @@ -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
>> 
>>