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

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Date:   Fri, 22 Sep 2023 09:00:45 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Baoquan He <bhe@...hat.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,
        patches@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/4] mm/slub: refactor calculate_order() and
 calc_slab_order()

On 9/16/23 03:28, Baoquan He wrote:
> On 09/08/23 at 04:53pm, Vlastimil Babka wrote:
>> @@ -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.

Maybe, although it's not a hot path. But we should make sure the
num_present_cpus() cannot change. Could it be e.g. low (1) very early when
we bootstrap the initial caches, but then update and at least most of the
caches then reflect the real number of cpus? With a static we would create
everything with 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
>> 
>