On 1/27/21 8:49 PM, Baolin Wang wrote:
>
>
> 在 2021/1/28 11:41, Jens Axboe 写道:
>> On 1/27/21 8:22 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
>>
>> Kind of ugly with the two clauses for dropping the blkcg lock, one
>> being a cpu_relax() and the other a resched. How about something
>> like this:
>>
>>
>> diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
>> index 031114d454a6..4221a1539391 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);
>>
>
> Looks better to me. Do I need resend with your suggestion? Thanks.
Probably best, gives Tejun another chance to sign off on it :-)
--
Jens Axboe
