Re: [PATCH v2 4/5] block, scsi: Rework runtime power management

[jianchao.wang]

On 07/27/2018 06:24 AM, Bart Van Assche wrote:
> On Thu, 2018-07-26 at 10:45 +0800, jianchao.wang wrote:
>> On 07/26/2018 06:26 AM, Bart Van Assche wrote:
>>> @@ -102,9 +109,11 @@ int blk_pre_runtime_suspend(struct request_queue *q)
>>>             return ret;
>>>  
>>>     blk_pm_runtime_lock(q);
>>> +   blk_set_preempt_only(q);
>>
>> We only stop non-RQF_PM request entering when RPM_SUSPENDING and 
>> RPM_RESUMING.
>> blk_pre_runtime_suspend should only _check_ whether runtime suspend is 
>> allowed.
>> So we should not set preempt_only here.
>>
>>> +   percpu_ref_switch_to_atomic_sync(&q->q_usage_counter);
>>>  
>>>     spin_lock_irq(q->queue_lock);
>>> -   if (q->nr_pending) {
>>> +   if (!percpu_ref_is_zero(&q->q_usage_counter)) {
>>>             ret = -EBUSY;
>>>             pm_runtime_mark_last_busy(q->dev);
>>>     } else {
>>> @@ -112,6 +121,7 @@ int blk_pre_runtime_suspend(struct request_queue *q)
>>>     }
>>>     spin_unlock_irq(q->queue_lock);
>>>  
>>> +   percpu_ref_switch_to_percpu(&q->q_usage_counter);
>>>     blk_pm_runtime_unlock(q);
> 
> Hello Jianchao,
> 
> There is only one caller of blk_post_runtime_suspend(), namely
> sdev_runtime_suspend(). That function calls pm->runtime_suspend() before
> it calls blk_post_runtime_suspend(). I think it would be wrong to set the
> PREEMPT_ONLY flag from inside blk_post_runtime_suspend() because that could
> cause pm->runtime_suspend() while a request is in progress.
> 

Hi Bart

If q_usage_counter is not zero here, we will leave the request_queue in preempt 
only mode.
The request_queue should be set to preempt only mode only when we confirm we 
could set
rpm_status to RPM_SUSPENDING or RPM_RESUMING.

Maybe we could set the PREEMPT_ONLY after we confirm we could set the rpm_status
to RPM_SUSPENDING.
Something like:

        percpu_ref_switch_to_atomic_sync(&q->q_usage_counter);
        if (!percpu_ref_is_zero(&q->q_usage_counter)) {
                ret = -EBUSY;
                pm_runtime_mark_last_busy(q->dev);
        } else {
                blk_set_preempt_only(q);
                if (!percpu_ref_is_zero(&q->q_usage_counter) {
                        ret = -EBUSY;
                        pm_runtime_mark_last_busy(q->dev);
                        blk_clear_preempt_only(q);
                } else {
                        q->rpm_status = RPM_SUSPENDIN;    
                }
        }


Thanks
Jianchao