Re: [PATCH v10 1/5] lib: objpool added: ring-array based lockless MPMC

[wuqiang.matt]

On 2023/10/15 23:43, Masami Hiramatsu (Google) wrote:
> On Sun, 15 Oct 2023 13:32:47 +0800
> "wuqiang.matt" <wuqiang.matt@xxxxxxxxxxxxx> 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