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Subject: Re: [PATCH v2] blk-cgroup: Use cond_resched() when destroy blkgs
To:     Baolin Wang <baolin.wang@linux.alibaba.com>, tj@kernel.org
Cc:     joseph.qi@linux.alibaba.com, linux-block@vger.kernel.org,
        cgroups@vger.kernel.org, linux-kernel@vger.kernel.org
References: <8e8a0c4644d5eb01b7f79ec9b67c2b240f4a6434.1611798287.git.baolin.wang@linux.alibaba.com>
 <3f3c12de-b5fb-a9ad-9324-55f5bf9d7453@kernel.dk>
 <f58ce890-d54c-e4a7-d379-ad4ad4ae20de@linux.alibaba.com>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <36d23f2a-4696-ff74-4423-6719b9670208@kernel.dk>
Date:   Wed, 27 Jan 2021 20:51:32 -0700
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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