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

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Message-ID: <5b9f5edbe26930765ee4adaa786db7da@codeaurora.org>
Date:   Mon, 08 Feb 2021 17:23:12 +0800
From:   Can Guo <cang@...eaurora.org>
To:     daejun7.park@...sung.com
Cc:     Greg KH <gregkh@...uxfoundation.org>, avri.altman@....com,
        jejb@...ux.ibm.com, martin.petersen@...cle.com,
        asutoshd@...eaurora.org, stanley.chu@...iatek.com,
        huobean@...il.com, bvanassche@....org,
        ALIM AKHTAR <alim.akhtar@...sung.com>,
        linux-scsi@...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: [PATCH v19 2/3] scsi: ufs: L2P map management for HPB read

On 2021-02-08 16:53, 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...

Can Guo.

> 
> int blk_queue_enter(struct request_queue *q, blk_mq_req_flags_t flags)
> {
> 	const bool pm = flags & BLK_MQ_REQ_PM;
> 
> 	while (true) {
> 		bool success = false;
> 
> 		rcu_read_lock();
> 		if (percpu_ref_tryget_live(&q->q_usage_counter)) {
> 			/*
> 			 * The code that increments the pm_only counter is
> 			 * responsible for ensuring that that counter is
> 			 * globally visible before the queue is unfrozen.
> 			 */
> 			if ((pm && queue_rpm_status(q) != RPM_SUSPENDED) ||
> 			    !blk_queue_pm_only(q)) {
> 				success = true;
> 			} else {
> 				percpu_ref_put(&q->q_usage_counter);
> 			}
> 		}
> 		rcu_read_unlock();
> 
> 		if (success)
> 			return 0;
> 
> 		if (flags & BLK_MQ_REQ_NOWAIT)
> 			return -EBUSY;	<-- out from the function.
> 
> Thanks,
> Daejun