Re: [PATCH v7] scsi: ufs: Quiesce all scsi devices before shutdown

[Stanley Chu <stanley.chu@mediatek.com>]

Hi Bart, Can, Chaotian,

Very appreciate your comments and suggestions, please see update below,

On Tue, 2020-08-04 at 00:04 +0800, Bart Van Assche wrote:
> On 2020-08-03 03:04, Stanley Chu wrote:
> > diff --git a/drivers/scsi/ufs/ufshcd.c b/drivers/scsi/ufs/ufshcd.c
> > index 307622284239..7cb220b3fde0 100644
> > --- a/drivers/scsi/ufs/ufshcd.c
> > +++ b/drivers/scsi/ufs/ufshcd.c
> > @@ -8640,6 +8640,7 @@ EXPORT_SYMBOL(ufshcd_runtime_idle);
> >  int ufshcd_shutdown(struct ufs_hba *hba)
> >  {
> >  	int ret = 0;
> > +	struct scsi_target *starget;
> >  
> >  	if (!hba->is_powered)
> >  		goto out;
> > @@ -8647,11 +8648,27 @@ int ufshcd_shutdown(struct ufs_hba *hba)
> >  	if (ufshcd_is_ufs_dev_poweroff(hba) && ufshcd_is_link_off(hba))
> >  		goto out;
> >  
> > -	if (pm_runtime_suspended(hba->dev)) {
> > -		ret = ufshcd_runtime_resume(hba);
> > -		if (ret)
> > -			goto out;
> > -	}
> > +	/*
> > +	 * Let runtime PM framework manage and prevent concurrent runtime
> > +	 * operations with shutdown flow.
> > +	 */
> > +	pm_runtime_get_sync(hba->dev);
> > +
> > +	/*
> > +	 * Quiesce all SCSI devices to prevent any non-PM requests sending
> > +	 * from block layer during and after shutdown.
> > +	 *
> > +	 * Here we can not use blk_cleanup_queue() since PM requests
> > +	 * (with BLK_MQ_REQ_PREEMPT flag) are still required to be sent
> > +	 * through block layer. Therefore SCSI command queued after the
> > +	 * scsi_target_quiesce() call returned will block until
> > +	 * blk_cleanup_queue() is called.
> > +	 *
> > +	 * Besides, scsi_target_"un"quiesce (e.g., scsi_target_resume) can
> > +	 * be ignored since shutdown is one-way flow.
> > +	 */
> > +	list_for_each_entry(starget, &hba->host->__targets, siblings)
> > +		scsi_target_quiesce(starget);
> >  
> >  	ret = ufshcd_suspend(hba, UFS_SHUTDOWN_PM);
> >  out:
> 
> This seems wrong to me. Since ufshcd_shutdown() shuts down the link I think
> it should call scsi_remove_device() instead of scsi_target_quiesce().

I tried many ways to come out the final solution. Currently two options
are considered,

== Option 1 ==
	pm_runtime_get_sync(hba->dev);

	shost_for_each_device(sdev, hba->host) {
		scsi_autopm_get_device(sdev);
		if (sdev == hba->sdev_ufs_device)
			scsi_device_quiesce(sdev);
		else
			scsi_remove_device(sdev);
	}

	ret = ufshcd_suspend(hba, UFS_SHUTDOWN_PM);

	scsi_remove_device(hba->sdev_ufs_device);

Note. Using scsi_autopm_get_device() instead of pm_runtime_disable()
is to prevent noisy message by below checking,

	WARN_ON_ONCE(sdev->quiesced_by && sdev->quiesced_by != current);

in
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/drivers/scsi/scsi_lib.c#n2515

This warning shows up if we try to quiesce a runtime-suspended SCSI
device. This is possible during our new shutdown flow. Using
scsi_autopm_get_device() to resume all SCSI devices first can prevent
it.

In addition, normally sd_shutdown() would be executed prior than
ufshcd_shutdown(). If scsi_remove_device() is invoked by
ufshcd_shutdown(), sd_shutdown() will be executed again for a SCSI disk
by

[  131.398977]  sd_shutdown+0x44/0x118
[  131.399416]  sd_remove+0x5c/0xc4
[  131.399824]  device_release_driver_internal+0x1c4/0x2e4
[  131.400481]  device_release_driver+0x18/0x24
[  131.401018]  bus_remove_device+0x108/0x134
[  131.401533]  device_del+0x2dc/0x630
[  131.401973]  __scsi_remove_device+0xc0/0x174
[  131.402510]  scsi_remove_device+0x30/0x48
[  131.403014]  ufshcd_shutdown+0xc8/0x138

In this case, we could see SYNCHRONIZE_CACHE command will be sent to the
same SCSI device twice. This is kind of wired during shutdown flow.

Moreover, in consideration of performance of ufshcd_shutdown(), Option 1
obviously degrades the latency a lot by scsi_remove_device(). Please see
the "Performance Measurement" data below.

Compared Option 2, this way is simpler and also effective. This way may
be a better compromise.

== Option 2  ==
	pm_runtime_get_sync(hba->dev);

	shost_for_each_device(sdev, hba->host) {
		scsi_autopm_get_device(sdev);
		scsi_device_quiesce(sdev);
	}

== Performance Measurement ==
As-Is: < 5 ms
Option 1: 850 ms
Option 2: 60 ms

What would you prefer? Or would you have any further suggestions?

Thanks,

Stanley Chu

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