blkcg: allocate struct blkcg_gq outside request queue spinlock
blkg_conf_prep() currently calls blkg_lookup_create() while holding
request queue spinlock. This means allocating memory for struct
blkcg_gq has to be made non-blocking. This causes occasional -ENOMEM
failures in call paths like below:
pcpu_alloc+0x68f/0x710
__alloc_percpu_gfp+0xd/0x10
__percpu_counter_init+0x55/0xc0
cfq_pd_alloc+0x3b2/0x4e0
blkg_alloc+0x187/0x230
blkg_create+0x489/0x670
blkg_lookup_create+0x9a/0x230
blkg_conf_prep+0x1fb/0x240
__cfqg_set_weight_device.isra.105+0x5c/0x180
cfq_set_weight_on_dfl+0x69/0xc0
cgroup_file_write+0x39/0x1c0
kernfs_fop_write+0x13f/0x1d0
__vfs_write+0x23/0x120
vfs_write+0xc2/0x1f0
SyS_write+0x44/0xb0
entry_SYSCALL_64_fastpath+0x18/0xad
In the code path above, percpu allocator cannot call vmalloc() due to
queue spinlock.
A failure in this call path gives grief to tools which are trying to
configure io weights. We see occasional failures happen shortly after
reboots even when system is not under any memory pressure. Machines
with a lot of cpus are more vulnerable to this condition.
Do struct blkcg_gq allocations outside the queue spinlock to allow
blocking during memory allocations.
Suggested-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Tahsin Erdogan <tahsin@google.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index bbe7ee0..7c29471 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -772,6 +772,27 @@ struct blkg_rwstat blkg_rwstat_recursive_sum(struct blkcg_gq *blkg,
}
EXPORT_SYMBOL_GPL(blkg_rwstat_recursive_sum);
+/* Performs queue bypass and policy enabled checks then looks up blkg. */
+static struct blkcg_gq *blkg_lookup_check(struct blkcg *blkcg,
+ const struct blkcg_policy *pol,
+ struct request_queue *q)
+{
+ WARN_ON_ONCE(!rcu_read_lock_held());
+ lockdep_assert_held(q->queue_lock);
+
+ if (!blkcg_policy_enabled(q, pol))
+ return ERR_PTR(-EOPNOTSUPP);
+
+ /*
+ * This could be the first entry point of blkcg implementation and
+ * we shouldn't allow anything to go through for a bypassing queue.
+ */
+ if (unlikely(blk_queue_bypass(q)))
+ return ERR_PTR(blk_queue_dying(q) ? -ENODEV : -EBUSY);
+
+ return __blkg_lookup(blkcg, q, true /* update_hint */);
+}
+
/**
* blkg_conf_prep - parse and prepare for per-blkg config update
* @blkcg: target block cgroup
@@ -789,6 +810,7 @@ int blkg_conf_prep(struct blkcg *blkcg, const struct blkcg_policy *pol,
__acquires(rcu) __acquires(disk->queue->queue_lock)
{
struct gendisk *disk;
+ struct request_queue *q;
struct blkcg_gq *blkg;
struct module *owner;
unsigned int major, minor;
@@ -807,44 +829,95 @@ int blkg_conf_prep(struct blkcg *blkcg, const struct blkcg_policy *pol,
if (!disk)
return -ENODEV;
if (part) {
- owner = disk->fops->owner;
- put_disk(disk);
- module_put(owner);
- return -ENODEV;
+ ret = -ENODEV;
+ goto fail;
}
+ q = disk->queue;
+
rcu_read_lock();
- spin_lock_irq(disk->queue->queue_lock);
+ spin_lock_irq(q->queue_lock);
- if (blkcg_policy_enabled(disk->queue, pol))
- blkg = blkg_lookup_create(blkcg, disk->queue);
- else
- blkg = ERR_PTR(-EOPNOTSUPP);
-
+ blkg = blkg_lookup_check(blkcg, pol, q);
if (IS_ERR(blkg)) {
ret = PTR_ERR(blkg);
- rcu_read_unlock();
- spin_unlock_irq(disk->queue->queue_lock);
- owner = disk->fops->owner;
- put_disk(disk);
- module_put(owner);
- /*
- * If queue was bypassing, we should retry. Do so after a
- * short msleep(). It isn't strictly necessary but queue
- * can be bypassing for some time and it's always nice to
- * avoid busy looping.
- */
- if (ret == -EBUSY) {
- msleep(10);
- ret = restart_syscall();
- }
- return ret;
+ goto fail_unlock;
}
+ if (blkg)
+ goto success;
+
+ /*
+ * Create blkgs walking down from blkcg_root to @blkcg, so that all
+ * non-root blkgs have access to their parents.
+ */
+ while (true) {
+ struct blkcg *pos = blkcg;
+ struct blkcg *parent;
+ struct blkcg_gq *new_blkg;
+
+ parent = blkcg_parent(blkcg);
+ while (parent && !__blkg_lookup(parent, q, false)) {
+ pos = parent;
+ parent = blkcg_parent(parent);
+ }
+
+ /* Drop locks to do new blkg allocation with GFP_KERNEL. */
+ spin_unlock_irq(q->queue_lock);
+ rcu_read_unlock();
+
+ new_blkg = blkg_alloc(pos, q, GFP_KERNEL);
+ if (unlikely(!new_blkg)) {
+ ret = -ENOMEM;
+ goto fail;
+ }
+
+ rcu_read_lock();
+ spin_lock_irq(q->queue_lock);
+
+ blkg = blkg_lookup_check(pos, pol, q);
+ if (IS_ERR(blkg)) {
+ ret = PTR_ERR(blkg);
+ goto fail_unlock;
+ }
+
+ if (blkg) {
+ blkg_free(new_blkg);
+ } else {
+ blkg = blkg_create(pos, q, new_blkg);
+ if (unlikely(IS_ERR(blkg))) {
+ ret = PTR_ERR(blkg);
+ goto fail_unlock;
+ }
+ }
+
+ if (pos == blkcg)
+ goto success;
+ }
+success:
ctx->disk = disk;
ctx->blkg = blkg;
ctx->body = body;
return 0;
+
+fail_unlock:
+ spin_unlock_irq(q->queue_lock);
+ rcu_read_unlock();
+fail:
+ owner = disk->fops->owner;
+ put_disk(disk);
+ module_put(owner);
+ /*
+ * If queue was bypassing, we should retry. Do so after a
+ * short msleep(). It isn't strictly necessary but queue
+ * can be bypassing for some time and it's always nice to
+ * avoid busy looping.
+ */
+ if (ret == -EBUSY) {
+ msleep(10);
+ ret = restart_syscall();
+ }
+ return ret;
}
EXPORT_SYMBOL_GPL(blkg_conf_prep);