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

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

[Baolin Wang]

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(), thus we can use cond_resched()
instead of cpu_relax() 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

Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
---
 block/blk-cgroup.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 3465d6e..af7c0ce 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -1028,7 +1028,7 @@ void blkcg_destroy_blkgs(struct blkcg *blkcg)
 			spin_unlock(&q->queue_lock);
 		} else {
 			spin_unlock_irq(&blkcg->lock);
-			cpu_relax();
+			cond_resched();
 			spin_lock_irq(&blkcg->lock);
 		}
 	}
-- 
1.8.3.1

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

[Tejun Heo]

Hello, Baolin.

On Tue, Jan 26, 2021 at 09:33:25PM +0800, 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(), thus we can use cond_resched()
> instead of cpu_relax() 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
> 
> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>

* Can you please add might_sleep() at the top of the function?

* Given that the system can accumulate a huge number of blkgs in
  pathological cases, I wonder whether a better way to go about it is
  explicitly testing need_resched() on each loop and release locks and do
  cond_resched() if true?

Thanks.

-- 
tejun

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

[Baolin Wang]

Hi Tejun,

> Hello, Baolin.
> 
> On Tue, Jan 26, 2021 at 09:33:25PM +0800, 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(), thus we can use cond_resched()
>> instead of cpu_relax() 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
>>
>> Signed-off-by: Baolin Wang <baolin.wang-KPsoFbNs7GizrGE5bRqYAgC/G2K4zDHf@public.gmane.org>
> 
> * Can you please add might_sleep() at the top of the function?

Sure.

> 
> * Given that the system can accumulate a huge number of blkgs in
>    pathological cases, I wonder whether a better way to go about it is
>    explicitly testing need_resched() on each loop and release locks and do
>    cond_resched() if true?

Yes, sound better to to me and will update in next version. Thanks for 
your sugestion.