Re: [PATCH 0/5] blk-mq: allow to run queue if queue refcount is held

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On 4/1/19 10:52 AM, Ming Lei wrote:
> On Sun, Mar 31, 2019 at 07:39:17PM -0700, Bart Van Assche wrote:
>> On 3/31/19 7:00 PM, Ming Lei wrote:
>>> On Sun, Mar 31, 2019 at 08:27:35AM -0700, Bart Van Assche wrote:
>>>> I'm not sure the approach of this patch series is really the direction we
>>>> should pursue. There are many block driver that free resources immediately
>>>
>>> Please see scsi_run_queue(), and the queue refcount is always held
>>> before run queue.
>>
>> That's not correct. There is no guarantee that q->q_usage_counter > 0 when
>> scsi_run_queue() is called from inside scsi_requeue_run_queue().
> 
> We don't need the guarantee of 'q->q_usage_counter > 0', I mean the
> queue's kobj reference counter.
> 
> What we need is to allow run queue to work correctly after queue is frozen
> or cleaned up.
> 
>>
>>>> I'd like to avoid having to modify all block drivers that free resources
>>>> immediately after blk_cleanup_queue() has returned. Have you considered to
>>>> modify blk_mq_run_hw_queues() such that it becomes safe to call that
>>>> function while blk_cleanup_queue() is in progress, e.g. by inserting a
>>>> percpu_ref_tryget_live(&q->q_usage_counter) /
>>>> percpu_ref_put(&q->q_usage_counter) pair?
>>>
>>> It can't work because blk_mq_run_hw_queues may happen after
>>> percpu_ref_exit() is done.
>>>
>>> However, if we move percpu_ref_exit() into queue's release handler, we
>>> don't need to grab q->q_usage_counter any more in blk_mq_run_hw_queues(),
>>> and we still have to free hw queue resources in queue's release handler,
>>> that is exactly what this patchset is doing.
>>>
>>> In short, getting q->q_usage_counter doesn't make a difference on this
>>> issue.
>>
>> percpu_ref_tryget_live() fails if a per-cpu counter is in the "dead" state.
>> percpu_ref_kill() changes the state of a per-cpu counter to the "dead"
>> state. blk_freeze_queue_start() calls percpu_ref_kill(). blk_cleanup_queue()
>> already calls blk_set_queue_dying() and that last function calls
>> blk_freeze_queue_start(). So I think that what you wrote is not correct and
>> that inserting a percpu_ref_tryget_live()/percpu_ref_put() pair in
>> blk_mq_run_hw_queues() or blk_mq_run_hw_queue() would make a difference and
>> also that moving the percpu_ref_exit() call into blk_release_queue() makes
>> sense.
> 
> If percpu_ref_exit() is moved to blk_release_queue(), we still need to
> move freeing of hw queue's resource into blk_release_queue() like what
> the patchset is doing.

Hi Ming,

Would you mind help explain why we still need to move freeing of hw queue's
resource into blk_release_queue() like what the patchset is doing?

Let's assume there is no deadlock when percpu_ref_tryget_live() is used,
blk_mq_run_hw_queues() would not be able to move forward as __PERCPU_REF_DEAD is
already set. Why we still need to move freeing of hw queue's resource into
blk_release_queue()?

Thank you very much!

Dongli Zhang

> 
> Then we don't need to get/put q_usage_counter in blk_mq_run_hw_queues() any more,
> do we?
> 
> 
> Thanks,
> Ming
> 



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