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Date:   Tue, 09 Feb 2021 10:28:01 +0800
From:   Can Guo <cang@codeaurora.org>
To:     daejun7.park@samsung.com
Cc:     Greg KH <gregkh@linuxfoundation.org>, avri.altman@wdc.com,
        jejb@linux.ibm.com, martin.petersen@oracle.com,
        asutoshd@codeaurora.org, stanley.chu@mediatek.com,
        huobean@gmail.com, bvanassche@acm.org,
        ALIM AKHTAR <alim.akhtar@samsung.com>,
        linux-scsi@vger.kernel.org, linux-kernel@vger.kernel.org,
        Sung-Jun Park <sungjun07.park@samsung.com>,
        yongmyung lee <ymhungry.lee@samsung.com>,
        Jinyoung CHOI <j-young.choi@samsung.com>,
        BoRam Shin <boram.shin@samsung.com>,
        SEUNGUK SHIN <seunguk.shin@samsung.com>
Subject: Re: [PATCH v19 2/3] scsi: ufs: L2P map management for HPB read
In-Reply-To: <20210209012734epcms2p8354347b1dd71601e74b505c715d36af0@epcms2p8>
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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