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Date: Fri, 19 Apr 2024 17:20:20 +0800
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Subject: Re: [PATCH] blk-iocost: do not WARNING if iocg has already offlined
To: Tejun Heo <tj@kernel.org>, linan666@huaweicloud.com
Cc: josef@toxicpanda.com, axboe@kernel.dk, hch@lst.de,
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References: <20240418072340.2090877-1-linan666@huaweicloud.com>
 <ZiFG5KtGGpYdOmnY@slm.duckdns.org>
From: Li Nan <linan666@huaweicloud.com>
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在 2024/4/19 0:14, Tejun Heo 写道:
> Hello,
> 
> On Thu, Apr 18, 2024 at 03:23:40PM +0800, linan666@huaweicloud.com wrote:
>> From: Li Nan <linan122@huawei.com>
>>
>> In iocg_pay_debt(), warn is triggered if 'active_list' is empty, which
>> is intended to confirm iocg is avitve when it has debt. However, warn
>> can be triggered during removing cgroup controller, as
> 
> Maybe saying "a blkcg is being removed" is clearer?

Thanks for your suggestion. I will correct my expression in next version.

> 
>> iocg_waitq_timer_fn() is awakened at that time.
>>
>>    WARNING: CPU: 0 PID: 2344971 at block/blk-iocost.c:1402 iocg_pay_debt+0x14c/0x190
>>    Call trace:
>>    iocg_pay_debt+0x14c/0x190
>>    iocg_kick_waitq+0x438/0x4c0
>>    iocg_waitq_timer_fn+0xd8/0x130
>>    __run_hrtimer+0x144/0x45c
>>    __hrtimer_run_queues+0x16c/0x244
>>    hrtimer_interrupt+0x2cc/0x7b0
>>
>> The warn in this situation is meaningless. Since this iocg is being
>> removed, the state of the 'active_list' is irrelevant, and 'waitq_timer'
>> is canceled after removing 'active_list' in ioc_pd_free(), which ensure
>> iocg is freed after iocg_waitq_timer_fn() returns.
>>
>> Therefore, add the check if iocg has already offlined to avoid warn
>> when removing cgroup controller.
>>
>> Signed-off-by: Li Nan <linan122@huawei.com>
>> ---
>>   block/blk-iocost.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/block/blk-iocost.c b/block/blk-iocost.c
>> index baa20c85799d..2e109c016a39 100644
>> --- a/block/blk-iocost.c
>> +++ b/block/blk-iocost.c
>> @@ -1440,7 +1440,7 @@ static void iocg_pay_debt(struct ioc_gq *iocg, u64 abs_vpay,
>>   	lockdep_assert_held(&iocg->waitq.lock);
>>   
>>   	/* make sure that nobody messed with @iocg */
>> -	WARN_ON_ONCE(list_empty(&iocg->active_list));
>> +	WARN_ON_ONCE(list_empty(&iocg->active_list) && iocg->pd.online);
> 
> Can you add a comment explaining why we need the pd.online test?

Yeah, I will add comment in next version.

> 
> Other than the above nits, looks great to me. Please feel free to add
> 
>    Acked-by: Tejun Heo <tj@kernel.org>
> 
> Thanks.
> 

-- 
Thanks,
Nan