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Date:   Mon, 16 Oct 2023 00:06:11 +0800
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To:     "Masami Hiramatsu (Google)" <mhiramat@kernel.org>
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References: <20231015053251.707442-1-wuqiang.matt@bytedance.com>
 <20231015053251.707442-2-wuqiang.matt@bytedance.com>
 <20231016004356.b5f3f815cb8d7c0994934332@kernel.org>
From:   "wuqiang.matt" <wuqiang.matt@bytedance.com>
Subject: Re: [PATCH v10 1/5] lib: objpool added: ring-array based lockless
 MPMC
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On 2023/10/15 23:43, Masami Hiramatsu (Google) wrote:
> On Sun, 15 Oct 2023 13:32:47 +0800
> "wuqiang.matt" <wuqiang.matt@bytedance.com> wrote:
> 
>> objpool is a scalable implementation of high performance queue for
>> object allocation and reclamation, such as kretprobe instances.
>>
>> With leveraging percpu ring-array to mitigate hot spots of memory
>> contention, it delivers near-linear scalability for high parallel
>> scenarios. The objpool is best suited for the following cases:
>> 1) Memory allocation or reclamation are prohibited or too expensive
>> 2) Consumers are of different priorities, such as irqs and threads
>>
>> Limitations:
>> 1) Maximum objects (capacity) is fixed after objpool creation
>> 2) All pre-allocated objects are managed in percpu ring array,
>>     which consumes more memory than linked lists
>>
> 
> Thanks for updating! This looks good to me except 2 points.
> 
> [...]
>> +
>> +/* initialize object pool and pre-allocate objects */
>> +int objpool_init(struct objpool_head *pool, int nr_objs, int object_size,
>> +		gfp_t gfp, void *context, objpool_init_obj_cb objinit,
>> +		objpool_fini_cb release)
>> +{
>> +	int rc, capacity, slot_size;
>> +
>> +	/* check input parameters */
>> +	if (nr_objs <= 0 || nr_objs > OBJPOOL_NR_OBJECT_MAX ||
>> +	    object_size <= 0 || object_size > OBJPOOL_OBJECT_SIZE_MAX)
>> +		return -EINVAL;
>> +
>> +	/* align up to unsigned long size */
>> +	object_size = ALIGN(object_size, sizeof(long));
>> +
>> +	/* calculate capacity of percpu objpool_slot */
>> +	capacity = roundup_pow_of_two(nr_objs);
> 
> This must be 'roundup_pow_of_two(nr_objs + 1)' because if nr_objs is power
> of 2 and all objects are pushed on the same slot, tail == head. This
> means empty and full is the same.

That won't happen. Would tail and head wrap only when >= 2^32. When all
objects are pushed to the same slot, tail will be (head + capacity).

> 
>> +	if (!capacity)
>> +		return -EINVAL;
>> +
>> +	/* initialize objpool pool */
>> +	memset(pool, 0, sizeof(struct objpool_head));
>> +	pool->nr_cpus = nr_cpu_ids;
>> +	pool->obj_size = object_size;
>> +	pool->capacity = capacity;
>> +	pool->gfp = gfp & ~__GFP_ZERO;
>> +	pool->context = context;
>> +	pool->release = release;
>> +	slot_size = pool->nr_cpus * sizeof(struct objpool_slot);
>> +	pool->cpu_slots = kzalloc(slot_size, pool->gfp);
>> +	if (!pool->cpu_slots)
>> +		return -ENOMEM;
>> +
>> +	/* initialize per-cpu slots */
>> +	rc = objpool_init_percpu_slots(pool, nr_objs, context, objinit);
>> +	if (rc)
>> +		objpool_fini_percpu_slots(pool);
>> +	else
>> +		refcount_set(&pool->ref, pool->nr_objs + 1);
>> +
>> +	return rc;
>> +}
>> +EXPORT_SYMBOL_GPL(objpool_init);
>> +
> 
> [...]
> 
>> +
>> +/* drop unused objects and defref objpool for releasing */
>> +void objpool_fini(struct objpool_head *pool)
>> +{
>> +	void *obj;
>> +
>> +	do {
>> +		/* grab object from objpool and drop it */
>> +		obj = objpool_pop(pool);
>> +
>> +		/*
>> +		 * drop reference of objpool anyway even if
>> +		 * the obj is NULL, since one extra ref upon
>> +		 * objpool was already grabbed during pool
>> +		 * initialization in objpool_init()
>> +		 */
>> +		if (refcount_dec_and_test(&pool->ref))
>> +			objpool_free(pool);
> 
> Nit: you can call objpool_drop() instead of repeating the same thing here.

objpool_drop won't deref objpool if given obj is NULL. But here we need
drop objpool anyway even if obj is NULL.

> Thank you,
> 
>> +	} while (obj);
>> +}
>> +EXPORT_SYMBOL_GPL(objpool_fini);
>> -- 
>> 2.40.1
>>

Thanks for you time