Re: [PATCH v2 3/3] blk-cgroup: move async bio punt state to blkcg
From: Nilay Shroff
Date: Wed Sep 16 2026 - 01:48:40 EST
On 9/15/26 9:24 PM, yu kuai wrote:
Hi,Ideally yes we would not enter into blkcg_css_free() until all
在 2026/9/13 21:29, Nilay Shroff 写道:
On 9/13/26 12:24 PM, Yu Kuai wrote:Ok.
From: Yu Kuai <yukuai@xxxxxxx>Instead of using guard(spinlock)(...) here, I think we could use the
blkcg_punt_bio_submit() currently queues punted bios on
blkg->async_bios,
so it has to call bio_blkg() to find or create a queue-local blkg. Bios
now carry and pin the blkcg css, so punted bio lifetime no longer
needs to
be anchored by a blkg.
Keeping the punt state in blkg can instantiate a blkg even when no blkcg
policy is enabled, just to bounce submission from a shared kthread.
Move
async_bio_lock, async_bios and async_bio_work to struct blkcg, and queue
punted bios on bio_blkcg() for non-root cgroups. Root or
unassociated bios
are submitted directly.
This preserves the priority-inversion avoidance while preventing
blkcg_punt_bio_submit() from creating blkgs that are not needed by any
policy.
Signed-off-by: Yu Kuai <yukuai@xxxxxxx>
---
block/blk-cgroup.c | 52 ++++++++++++++++++++++++++--------------------
block/blk-cgroup.h | 14 ++++++-------
2 files changed, 35 insertions(+), 31 deletions(-)
diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 59ccfefe16a8..aa3cee107ebe 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -180,14 +180,10 @@ static void blkg_free(struct blkcg_gq *blkg)
static void __blkg_release(struct rcu_head *rcu)
{
struct blkcg_gq *blkg = container_of(rcu, struct blkcg_gq,
rcu_head);
-#ifdef CONFIG_BLK_CGROUP_PUNT_BIO
- WARN_ON(!bio_list_empty(&blkg->async_bios));
-#endif
-
blkg_free(blkg);
}
/*
* A group is RCU protected, but having an rcu lock does not mean
that one
@@ -226,23 +222,23 @@ static void blkg_release(struct percpu_ref *ref)
}
#ifdef CONFIG_BLK_CGROUP_PUNT_BIO
static struct workqueue_struct *blkcg_punt_bio_wq;
-static void blkg_async_bio_workfn(struct work_struct *work)
+static void blkcg_async_bio_workfn(struct work_struct *work)
{
- struct blkcg_gq *blkg = container_of(work, struct blkcg_gq,
- async_bio_work);
+ struct blkcg *blkcg = container_of(work, struct blkcg,
async_bio_work);
struct bio_list bios = BIO_EMPTY_LIST;
struct bio *bio;
struct blk_plug plug;
bool need_plug = false;
- /* as long as there are pending bios, @blkg can't go away */
- spin_lock(&blkg->async_bio_lock);
- bio_list_merge_init(&bios, &blkg->async_bios);
- spin_unlock(&blkg->async_bio_lock);
+ /* as long as there are pending bios, @blkcg can't go away */
+ {
+ guard(spinlock)(&blkcg->async_bio_lock);
+ bio_list_merge_init(&bios, &blkcg->async_bios);
+ }
simpler spin_lock()/spin_unlock() helpers. IMO, they are easier
to read and reason about for these short critical sections.
/* start plug only when bio_list contains at least 2 bios */
if (bios.head && bios.head->bi_next) {
need_plug = true;
blk_start_plug(&plug);
@@ -259,19 +255,20 @@ static void blkg_async_bio_workfn(struct
work_struct *work)
* cgroup. Use this helper instead of submit_bio to punt the
actual issuing to
* a dedicated per-blkcg work item to avoid such priority inversions.
*/
void blkcg_punt_bio_submit(struct bio *bio)
{
- struct blkcg_gq *blkg = bio_blkg(bio);
+ struct blkcg *blkcg = bio_blkcg(bio);
- if (blkg && blkg->parent) {
- spin_lock(&blkg->async_bio_lock);
- bio_list_add(&blkg->async_bios, bio);
- spin_unlock(&blkg->async_bio_lock);
- queue_work(blkcg_punt_bio_wq, &blkg->async_bio_work);
+ if (blkcg && cgroup_parent(blkcg->css.cgroup)) {
+ {
+ guard(spinlock)(&blkcg->async_bio_lock);
+ bio_list_add(&blkcg->async_bios, bio);
+ }
+ queue_work(blkcg_punt_bio_wq, &blkcg->async_bio_work);
Again same here, replace guard() with spin_lock() and spin_unlock()
helpers.
} else {
- /* Never bounce if there is no non-root blkg to queue on. */
+ /* Never bounce if there is no non-root blkcg to queue on. */
submit_bio(bio);
}
}
EXPORT_SYMBOL_GPL(blkcg_punt_bio_submit);
@@ -350,15 +347,10 @@ static struct blkcg_gq *blkg_alloc(struct
blkcg *blkcg, struct gendisk *disk,
blkg->q = disk->queue;
INIT_LIST_HEAD(&blkg->q_node);
blkg->blkcg = blkcg;
blkg->blkcg_id = blkcg->css.id;
blkg->iostat.blkg = blkg;
-#ifdef CONFIG_BLK_CGROUP_PUNT_BIO
- spin_lock_init(&blkg->async_bio_lock);
- bio_list_init(&blkg->async_bios);
- INIT_WORK(&blkg->async_bio_work, blkg_async_bio_workfn);
-#endif
u64_stats_init(&blkg->iostat.sync);
for_each_possible_cpu(cpu) {
u64_stats_init(&per_cpu_ptr(blkg->iostat_cpu, cpu)->sync);
per_cpu_ptr(blkg->iostat_cpu, cpu)->blkg = blkg;
@@ -1399,10 +1391,16 @@ static void blkcg_css_free(struct
cgroup_subsys_state *css)
if (blkcg->cpd[i])
blkcg_policy[i]->cpd_free_fn(blkcg->cpd[i]);
mutex_unlock(&blkcg_pol_mutex);
+#ifdef CONFIG_BLK_CGROUP_PUNT_BIO
+ {
+ guard(spinlock)(&blkcg->async_bio_lock);
+ WARN_ON(!bio_list_empty(&blkcg->async_bios));
+ }
+#endif
This is a slightly different case. At this point blkcg_css_free() is
freeing the blkcg object after its final reference has gone away, so
there should be no concurrent context accessing blkcg->async_bios.
Therefore, I don't think we need to acquire async_bio_lock here just
to perform the WARN_ON() check.
Perhaps is it better just to remove the check? blkcg will be pinned
by any bio inside the list, so I believe this is safe.
references to blkcg are dropped. So the WARN_ON() appears to be
used just as a paranoia check. I'm okay either to drop it or
if you want to keep it then I suggest replacing bio_list_empty()
with bio_list_empty_careful(), so that the check explicitly allows
lockless inspection during teardown.
Thanks,
--Nilay