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SubjectRe: [PATCH v19 2/3] scsi: ufs: L2P map management for HPB read
On 2021-02-09 09:27, Daejun Park wrote:
>>>>>> @@ -342,13 +1208,14 @@ void ufshpb_suspend(struct ufs_hba *hba)
>>>>>> > struct scsi_device *sdev;
>>>>>> >
>>>>>> > shost_for_each_device(sdev, hba->host) {
>>>>>> > - hpb = sdev->hostdata;
>>>>>> > + hpb = ufshpb_get_hpb_data(sdev);
>>>>>> > if (!hpb)
>>>>>> > continue;
>>>>>> >
>>>>>> > if (ufshpb_get_state(hpb) != HPB_PRESENT)
>>>>>> > continue;
>>>>>> > ufshpb_set_state(hpb, HPB_SUSPEND);
>>>>>> > + ufshpb_cancel_jobs(hpb);
>>>>>>
>>>>>> Here may have a dead lock problem - in the case of runtime
>>>>>> suspend,
>>>>>> when ufshpb_suspend() is invoked, all of hba's children scsi
>>>>>> devices
>>>>>> are in RPM_SUSPENDED state. When this line tries to cancel a
>>>>>> running
>>>>>> map work, i.e. when ufshpb_get_map_req() calls below lines, it
>>>>>> will
>>>>>> be stuck at blk_queue_enter().
>>>>>>
>>>>>> req = blk_get_request(hpb->sdev_ufs_lu->request_queue,
>>>>>> REQ_OP_SCSI_IN, 0);
>>>>>>
>>>>>> Please check block layer power management, and see also commit
>>>>>> d55d15a33
>>>>>> ("scsi: block: Do not accept any requests while suspended").
>>>>>
>>>>> I am agree with your comment.
>>>>> How about add BLK_MQ_REQ_NOWAIT flag on blk_get_request() to avoid
>>>>> hang?
>>>>>
>>>>
>>>> That won't work - BLK_MQ_REQ_NOWAIT allows one to fast fail from
>>>> blk_mq_get_tag(),
>>>> but blk_queue_enter() comes before __blk_mq_alloc_request();
>>>>
>>> In blk_queue_enter(), BLK_MQ_REQ_NOWAIT flag can make error than wait
>>> rpm
>>> resume. Please refer following code.
>>
>> Oops, sorry, my memory needs to be refreshed on that part.
>>
>> But will BLK_MQ_REQ_NOWAIT flag breaks your original purpose? When
>> runtime suspend is out of the picture, if traffic is heavy on the
>> request queue, map_work() will be stopped frequently once it is
>> not able to get a request from the queue - that shall pull down the
>> efficiency of one map_work(), that may hurt random performance...
>
> I think deadlock prevention is the most important. So I want to add
> BLK_MQ_REQ_NOWAIT flag.
> Starvation of map request can be distinguish by return value of
> blk_get_request(). -EWOULDBLOCK means there is no available tags for
> this
> request. -EBUSY means failed on blk_queue_enter(). To overcome
> starvation
> of map request, we can try N times in heavy traffic situation (maybe
> N=3?).
>

LGTM. You make the call.

Regards,
Can Guo.

> Thanks,
> Daejun

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