From mboxrd@z Thu Jan  1 00:00:00 1970
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Date: Wed, 15 Aug 2018 19:23:04 +0800
From: Ming Lei <ming.lei@redhat.com>
To: "jianchao.wang" <jianchao.w.wang@oracle.com>
Cc: Jens Axboe <axboe@kernel.dk>, linux-block@vger.kernel.org,
	Alan Stern <stern@rowland.harvard.edu>,
	Christoph Hellwig <hch@lst.de>,
	Bart Van Assche <bart.vanassche@wdc.com>,
	Hannes Reinecke <hare@suse.de>,
	Johannes Thumshirn <jthumshirn@suse.de>,
	Adrian Hunter <adrian.hunter@intel.com>,
	"James E.J. Bottomley" <jejb@linux.vnet.ibm.com>,
	"Martin K. Petersen" <martin.petersen@oracle.com>,
	linux-scsi@vger.kernel.org
Subject: Re: [RFC PATCH V2 16/17] block: simplify runtime PM support
Message-ID: <20180815112302.GA27213@ming.t460p>
References: <20180811071220.357-1-ming.lei@redhat.com>
 <20180811071220.357-17-ming.lei@redhat.com>
 <6baaef0e-6845-d3d5-6d78-85ce89bc0a2a@oracle.com>
 <20180815082803.GA26016@ming.t460p>
 <bec354c7-3c62-e55e-453b-a1916001e313@oracle.com>
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In-Reply-To: <bec354c7-3c62-e55e-453b-a1916001e313@oracle.com>
List-ID: <linux-block@vger.kernel.org>

On Wed, Aug 15, 2018 at 05:47:57PM +0800, jianchao.wang wrote:
> 
> 
> On 08/15/2018 04:28 PM, Ming Lei wrote:
> > On Wed, Aug 15, 2018 at 02:39:05PM +0800, jianchao.wang wrote:
> >> Hi Ming
> >>
> >> On 08/11/2018 03:12 PM, Ming Lei wrote:
> >>> @@ -3786,6 +3800,10 @@ int blk_pre_runtime_suspend(struct request_queue *q)
> >>>  		q->rpm_status = RPM_SUSPENDING;
> >>>  	}
> >>>  	spin_unlock_irq(q->queue_lock);
> >>> +
> >>> +	if (!ret)
> >>> +		blk_freeze_queue_lock(q);
> >>> +
> >>>  	return ret;
> >>
> >> Requests seem to be able enter the request_queue while the rpm_status is RPM_SUSPENDING
> >> and miss the rpm_resume.
> > 
> > Any requests which enters queue during RPM_SUSPENDING will be drained by
> > blk_freeze_queue_lock().
> > 
> >>
> >> Even though blk_freeze_queue_lock will drain them before the real suspend start, but looks
> >> like we should stop the suspend at the moment.
> > 
> > Good point, even though it doesn't affect the correctness of this runtime PM
> > implementation.
> > 
> > We may improve this situation by the following way, what do you think of
> > it?
> > 
> > diff --git a/block/blk-core.c b/block/blk-core.c
> > index f42197c9f7af..84e1e6c7db87 100644
> > --- a/block/blk-core.c
> > +++ b/block/blk-core.c
> > @@ -3803,10 +3803,13 @@ int blk_pre_runtime_suspend(struct request_queue *q)
> >  {
> >  	int ret = 0;
> >  	bool busy = true;
> > +	unsigned long last_busy;
> >  
> >  	if (!q->dev)
> >  		return ret;
> >  
> > +	last_busy = READ_ONCE(dev->power.last_busy);
> > +
> >  	if (q->mq_ops)
> >  		busy = blk_mq_pm_queue_busy(q);
> >  
> > @@ -3823,6 +3826,13 @@ int blk_pre_runtime_suspend(struct request_queue *q)
> >  	if (!ret)
> >  		blk_freeze_queue_lock(q);
> >  
> > +	/*
> > +	 * Any new IO during this window will prevent the current suspend
> > +	 * from going on
> > +	 */
> > +	if (unlikely(last_busy != READ_ONCE(dev->power.last_busy)))
> > +		ret = -EBUSY;
> > +
> >  	return ret;
> >  }
> >  EXPORT_SYMBOL(blk_pre_runtime_suspend);
> > 
> 
> Wow, this is great method !
> Maybe we should add checking as following:
> 
> 
> +	last_busy = READ_ONCE(dev->power.last_busy);
> +       if (last_busy == jiffies)
> +           return -EBUSY

We may do that, but seems not necessary given the .runtime_suspend()
(scsi_runtime_suspend) is just called when the timer is expired.

As we know, even there is request entering queue during the suspend
window, not a big deal, the current approach still works fine.

Thanks,
Ming