2021-01-28 06:01:27

by Baolin Wang

Subject: [PATCH v3] blk-cgroup: Use cond_resched() when destroy blkgs

On !PREEMPT kernel, we can get below softlockup when doing stress
testing with creating and destroying block cgroup repeatly. The
reason is it may take a long time to acquire the queue's lock in
the loop of blkcg_destroy_blkgs(), or the system can accumulate a
huge number of blkgs in pathological cases. We can add a need_resched()
check on each loop and release locks and do cond_resched() if true
to avoid this issue, since the blkcg_destroy_blkgs() is not called
from atomic contexts.

[ 4757.010308] watchdog: BUG: soft lockup - CPU#11 stuck for 94s!
[ 4757.010698] Call trace:
[ 4757.010700]  blkcg_destroy_blkgs+0x68/0x150
[ 4757.010701]  cgwb_release_workfn+0x104/0x158
[ 4757.010702]  process_one_work+0x1bc/0x3f0
[ 4757.010704]  worker_thread+0x164/0x468
[ 4757.010705]  kthread+0x108/0x138

Suggested-by: Tejun Heo <[email protected]>
Signed-off-by: Baolin Wang <[email protected]>
---
Changes from v2:
- Simplify logics suggested by Jens.

Changes from v1:
- Add might_sleep() in blkcg_destroy_blkgs().
- Add an explicitly need_resched() check before releasing lock.
- Add some comments.
---
block/blk-cgroup.c | 18 +++++++++++++-----
1 file changed, 13 insertions(+), 5 deletions(-)

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 02ce205..4b4fcb5 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -1016,6 +1016,8 @@ static void blkcg_css_offline(struct cgroup_subsys_state *css)
*/
void blkcg_destroy_blkgs(struct blkcg *blkcg)
{
+ might_sleep();
+
spin_lock_irq(&blkcg->lock);

while (!hlist_empty(&blkcg->blkg_list)) {
@@ -1023,14 +1025,20 @@ void blkcg_destroy_blkgs(struct blkcg *blkcg)
struct blkcg_gq, blkcg_node);
struct request_queue *q = blkg->q;

- if (spin_trylock(&q->queue_lock)) {
- blkg_destroy(blkg);
- spin_unlock(&q->queue_lock);
- } else {
+ if (need_resched() || !spin_trylock(&q->queue_lock)) {
+ /*
+ * Given that the system can accumulate a huge number
+ * of blkgs in pathological cases, check to see if we
+ * need to rescheduling to avoid softlockup.
+ */
spin_unlock_irq(&blkcg->lock);
- cpu_relax();
+ cond_resched();
spin_lock_irq(&blkcg->lock);
+ continue;
}
+
+ blkg_destroy(blkg);
+ spin_unlock(&q->queue_lock);
}

spin_unlock_irq(&blkcg->lock);
--
1.8.3.1

2021-01-28 14:36:31

by Jens Axboe

Subject: Re: [PATCH v3] blk-cgroup: Use cond_resched() when destroy blkgs

On 1/27/21 10:58 PM, Baolin Wang wrote:
> On !PREEMPT kernel, we can get below softlockup when doing stress
> testing with creating and destroying block cgroup repeatly. The
> reason is it may take a long time to acquire the queue's lock in
> the loop of blkcg_destroy_blkgs(), or the system can accumulate a
> huge number of blkgs in pathological cases. We can add a need_resched()
> check on each loop and release locks and do cond_resched() if true
> to avoid this issue, since the blkcg_destroy_blkgs() is not called
> from atomic contexts.
>
> [ 4757.010308] watchdog: BUG: soft lockup - CPU#11 stuck for 94s!
> [ 4757.010698] Call trace:
> [ 4757.010700]  blkcg_destroy_blkgs+0x68/0x150
> [ 4757.010701]  cgwb_release_workfn+0x104/0x158
> [ 4757.010702]  process_one_work+0x1bc/0x3f0
> [ 4757.010704]  worker_thread+0x164/0x468
> [ 4757.010705]  kthread+0x108/0x138

Thanks, applied.

--
Jens Axboe