Re: [PATCH -next 2/2] block, bfq: make bfq_has_work() more accurate

["yukuai (C)" <yukuai3@huawei.com>]

在 2022/05/13 1:10, Jan Kara 写道:
> On Thu 12-05-22 09:30:16, yukuai (C) wrote:
>> On 2022/05/11 22:08, Jan Kara wrote:
>>> On Tue 10-05-22 21:16:29, Yu Kuai wrote:
>>>> bfq_has_work() is using busy_queues currently, which is not accurate
>>>> because bfq_queue is busy doesn't represent that it has requests. Since
>>>> bfqd aready has a counter 'queued' to record how many requests are in
>>>> bfq, use it instead of busy_queues.
>>>>
>>>> Noted that bfq_has_work() can be called with 'bfqd->lock' held, thus the
>>>> lock can't be held in bfq_has_work() to protect 'bfqd->queued'.
>>>>
>>>> Signed-off-by: Yu Kuai <yukuai3@huawei.com>
>>>
>>> So did you find this causing any real problem? Because bfq queue is
>>> accounted among busy queues once bfq_add_bfqq_busy() is called. And that
>>> happens once a new request is inserted into the queue so it should be very
>>> similar to bfqd->queued.
>>>
>>> 								Honza
>>
>> Hi,
>>
>> The related problem is described here:
>>
>> https://lore.kernel.org/all/20220510112302.1215092-1-yukuai3@huawei.com/
>>
>> The root cause of the panic is a linux-block problem, however, it can
>> be bypassed if bfq_has_work() is accurate. On the other hand,
>> unnecessary run_work will be triggered if bfqq stays busy:
>>
>> __blk_mq_run_hw_queue
>>   __blk_mq_sched_dispatch_requests
>>    __blk_mq_do_dispatch_sched
>>     if (!bfq_has_work())
>>      break;
>>     blk_mq_delay_run_hw_queues -> run again after 3ms
> 
> Ah, I see. So it is the other way around than I thought. Due to idling
> bfq_tot_busy_queues() can be greater than 0 even if there are no requests
> to dispatch. Indeed. OK, the patch makes sense. But please use WRITE_ONCE
> for the updates of bfqd->queued. Otherwise the READ_ONCE does not really
> make sense (it can still result in some bogus value due to compiler
> optimizations on the write side).

Thanks for you adivce, I'll send a new version.

Kuai