linux-kernel - RE: Re: [PATCH v19 2/3] scsi: ufs: L2P map management for HPB read

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Message-ID: <20210209012734epcms2p8354347b1dd71601e74b505c715d36af0@epcms2p8>
Date:   Tue, 09 Feb 2021 10:27:34 +0900
From:   Daejun Park <daejun7.park@...sung.com>
To:     Can Guo <cang@...eaurora.org>,
        Daejun Park <daejun7.park@...sung.com>
CC:     Greg KH <gregkh@...uxfoundation.org>,
        "avri.altman@....com" <avri.altman@....com>,
        "jejb@...ux.ibm.com" <jejb@...ux.ibm.com>,
        "martin.petersen@...cle.com" <martin.petersen@...cle.com>,
        "asutoshd@...eaurora.org" <asutoshd@...eaurora.org>,
        "stanley.chu@...iatek.com" <stanley.chu@...iatek.com>,
        "huobean@...il.com" <huobean@...il.com>,
        "bvanassche@....org" <bvanassche@....org>,
        ALIM AKHTAR <alim.akhtar@...sung.com>,
        "linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Sung-Jun Park <sungjun07.park@...sung.com>,
        yongmyung lee <ymhungry.lee@...sung.com>,
        Jinyoung CHOI <j-young.choi@...sung.com>,
        BoRam Shin <boram.shin@...sung.com>,
        SEUNGUK SHIN <seunguk.shin@...sung.com>
Subject: RE: Re: [PATCH v19 2/3] scsi: ufs: L2P map management for HPB read

>>>>> @@ -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?).

Thanks,
Daejun