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



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.



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