linux-kernel - Re: [PATCH] scsi: sd: add runtime pm to open / release

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Message-ID: <20200630193802.GA463609@rowland.harvard.edu>
Date:   Tue, 30 Jun 2020 15:38:02 -0400
From:   Alan Stern <stern@...land.harvard.edu>
To:     Bart Van Assche <bvanassche@....org>
Cc:     Martin Kepplinger <martin.kepplinger@...i.sm>, jejb@...ux.ibm.com,
        Can Guo <cang@...eaurora.org>, martin.petersen@...cle.com,
        linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel@...i.sm
Subject: Re: [PATCH] scsi: sd: add runtime pm to open / release

On Tue, Jun 30, 2020 at 12:23:12PM -0700, Bart Van Assche wrote:
> On 2020-06-30 11:02, Alan Stern wrote:
> > Right now there doesn't seem to be any mechanism for resuming the queue 
> > if an REQ_PREEMPT request is added while the queue is suspended.
> 
> I do not agree with the above statement. My understanding is that resuming
> happens as follows if a request is submitted against a runtime suspended
> queue owned by a SCSI LLD:
> 
> blk_queue_enter()
>   -> blk_pm_request_resume()

Assume that BLK_MQ_REQ_PREEMPT is set in flags.  Then where exactly 
does blk_queue_enter(q, flags) call blk_pm_request_resume(q)?

Nowhere, as far as I can see.

Alan Stern

>     -> pm_request_resume(dev)
>       -> __pm_runtime_resume(dev, RPM_ASYNC)
>         -> rpm_resume(dev, RPM_ASYNC)
>           -> dev->power.request = RPM_REQ_RESUME;
>           -> queue_work(pm_wq, &dev->power.work)
>             -> pm_runtime_work()
>               -> rpm_resume(dev, RPM_NOWAIT)
>                 -> callback = scsi_runtime_resume;
>                 -> rpm_callback(callback, dev);
>                   -> scsi_runtime_resume(dev);
>                     -> sdev_runtime_resume(dev);
>                       -> blk_pre_runtime_resume(sdev->request_queue);
>                         -> q->rpm_status = RPM_RESUMING;
>                       -> sd_resume(dev);
>                         -> sd_start_stop_device(sdkp);
>                           -> sd_pm_ops.runtime_resume == scsi_execute(sdp, START);
>                             -> blk_get_request(..., ..., BLK_MQ_REQ_PREEMPT)
>                               -> blk_mq_alloc_request()
>                                 -> blk_queue_enter()
>                                 -> __blk_mq_alloc_request()
>                             -> blk_execute_rq()
>                             -> blk_put_request()
>                       -> blk_post_runtime_resume(sdev->request_queue);
>                         -> q->rpm_status = RPM_ACTIVE;
>                         -> pm_runtime_mark_last_busy(q->dev);
>                         -> pm_request_autosuspend(q->dev);
> 
> Bart.