[PATCH 6/8]Re: bio-cgroup: The body of bio-cgroup

[Takuya Yoshikawa]

Hi i/o controller/scheduler developers,

Hirokazu Takahashi wrote:
> Hi,
> 
> As you pointed out, cgroup iocontext stuff isn't well designed yet
> since the current implementation of dm-iband doesn't need it.
> I'd like to leave the iocontext stuff to you I/O scheduler guys
> if you want to implement the bio-cgroup infrastructure to handle iocotexts
> as the I/O schedulers expect.

Yes, I want to contribute to this because I think the io context tracking part
can be used for a lot of other things. So it would be nice if we can talk about
"the most useful desing for everyone."

> 
>> Hi,
>>
>> Ryo Tsuruta wrote:
>>>  
>>> +void init_io_context(struct io_context *ioc)
>>> +{
>>> +	atomic_set(&ioc->refcount, 1);
>>> +	atomic_set(&ioc->nr_tasks, 1);
>>> +	spin_lock_init(&ioc->lock);
>>> +	ioc->ioprio_changed = 0;
>>> +	ioc->ioprio = 0;
>>> +	ioc->last_waited = jiffies; /* doesn't matter... */
>>> +	ioc->nr_batch_requests = 0; /* because this is 0 */
>>> +	ioc->aic = NULL;
>>> +	INIT_RADIX_TREE(&ioc->radix_root, GFP_ATOMIC | __GFP_HIGH);
>>> +	INIT_HLIST_HEAD(&ioc->cic_list);
>>> +	ioc->ioc_data = NULL;
>>> +}
>>> +
>>>  struct io_context *alloc_io_context(gfp_t gfp_flags, int node)
>>>  {
>>>  	struct io_context *ret;
>>>  
>>>  	ret = kmem_cache_alloc_node(iocontext_cachep, gfp_flags, node);
>>> -	if (ret) {
>>> -		atomic_set(&ret->refcount, 1);
>>> -		atomic_set(&ret->nr_tasks, 1);
>>> -		spin_lock_init(&ret->lock);
>>> -		ret->ioprio_changed = 0;
>>> -		ret->ioprio = 0;
>>> -		ret->last_waited = jiffies; /* doesn't matter... */
>>> -		ret->nr_batch_requests = 0; /* because this is 0 */
>>> -		ret->aic = NULL;
>>> -		INIT_RADIX_TREE(&ret->radix_root, GFP_ATOMIC | __GFP_HIGH);
>>> -		INIT_HLIST_HEAD(&ret->cic_list);
>>> -		ret->ioc_data = NULL;
>>> -	}
>>> +	if (ret)
>>> +		init_io_context(ret);
>>>  
>>>  	return ret;
>>>  }
>>
>>> +
>>> +/* Create a new bio-cgroup. */
>>> +static struct cgroup_subsys_state *
>>> +bio_cgroup_create(struct cgroup_subsys *ss, struct cgroup *cgrp)
>>> +{
>>> +	struct bio_cgroup *biog;
>>> +	struct io_context *ioc;
>>> +	int ret;
>>> +
>>> +	if (!cgrp->parent) {
>>> +		biog = &default_bio_cgroup;
>>> +		init_io_context(biog->io_context);
>>> +		/* Increment the referrence count not to be released ever. */
>>> +		atomic_inc(&biog->io_context->refcount);
>>> +		idr_init(&bio_cgroup_id);
>>> +		return &biog->css;
>>> +	}
>>> +
>>> +	biog = kzalloc(sizeof(*biog), GFP_KERNEL);
>>> +	ioc = alloc_io_context(GFP_KERNEL, -1);
>>> +	if (!ioc || !biog) {
>>> +		ret = -ENOMEM;
>>> +		goto out_err;
>>> +	}
>>> +	biog->io_context = ioc;
>> If I understand correctly io_contexts allocated by alloc_io_context() should
>> be owned by some tasks. In this case, the newly created io_context has no
>> owner though biog->io_context->nr_tasks == 1. With the cfq scheduler this may
>> cause some problems especially when cic_free_func() is called because
>> cfq_exit_io_context() would never be called. Am I wrong?
> 
> I think iocontext allocated for a certain cgroup should be owned by the
> cgroup itself, whose code isn't implemented yet. I think you need to
> enhance it a bit if the owner is a cgroup.
> 
>>> +retry:
>>> +	if (!idr_pre_get(&bio_cgroup_id, GFP_KERNEL)) {
>>> +		ret = -EAGAIN;
>>> +		goto out_err;
>>> +	}
>>> +	spin_lock_irq(&bio_cgroup_idr_lock);
>>> +	ret = idr_get_new_above(&bio_cgroup_id, (void *)biog, 1, &biog->id);
>>> +	spin_unlock_irq(&bio_cgroup_idr_lock);
>>> +	if (ret == -EAGAIN)
>>> +		goto retry;
>>> +	else if (ret)
>>> +		goto out_err;
>>> +
>>> +	return &biog->css;
>>> +out_err:
>>> +	if (biog)
>>> +		kfree(biog);
>>> +	if (ioc)
>>> +		put_io_context(ioc);
>>> +	return ERR_PTR(ret);
>>> +}
>>> +
>>> +/* Delete the bio-cgroup. */
>>> +static void bio_cgroup_destroy(struct cgroup_subsys *ss, struct cgroup *cgrp)
>>> +{
>>> +	struct bio_cgroup *biog = cgroup_bio(cgrp);
>>> +
>>> +	put_io_context(biog->io_context);
>> Here, I suspects what will happen under the following condition:
>> biog->io_context->refcount: 1 --> 0
>> biog->io_context->nr_tasks: 1 --> 1
>> ==> cfq_dtor() will be called but cfq_exit() has not be called yet.
>>
>>> +
>>> +	spin_lock_irq(&bio_cgroup_idr_lock);
>>> +	idr_remove(&bio_cgroup_id, biog->id);
>>> +	spin_unlock_irq(&bio_cgroup_idr_lock);
>>> +
>>> +	kfree(biog);
>>> +}
>>> +
>>> +
>>> +/* Determine the iocontext of the bio-cgroup that issued a given bio. */
>>> +struct io_context *get_bio_cgroup_iocontext(struct bio *bio)
>>> +{
>>> +	struct bio_cgroup *biog = NULL;
>>> +	struct io_context *ioc;
>>> +	int	id = 0;
>>> +
>>> +	id = get_bio_cgroup_id(bio);
>>> +	if (id)
>>> +		biog = find_bio_cgroup(id);
>>> +	if (!biog)
>>> +		biog = &default_bio_cgroup;
>>> +	ioc = biog->io_context;	/* default io_context for this cgroup */
>>> +	atomic_inc(&ioc->refcount);
>>> +	return ioc;
>>> +}
>>> +EXPORT_SYMBOL(get_bio_cgroup_iocontext);
>>
>> Thanks,
>> Takuya Yoshikawa
> 
> Thank you,
> Hirokazu Takahashi.
>