Re: [PATCH v5 5/7] block: Delay default elevator initialization

[Eric Farman <farman@linux.ibm.com>]

On 9/5/19 5:51 AM, Damien Le Moal wrote:
> When elevator_init_mq() is called from blk_mq_init_allocated_queue(),
> the only information known about the device is the number of hardware
> queues as the block device scan by the device driver is not completed
> yet for most drivers. The device type and elevator required features
> are not set yet, preventing to correctly select the default elevator
> most suitable for the device.
> 
> This currently affects all multi-queue zoned block devices which default
> to the "none" elevator instead of the required "mq-deadline" elevator.
> These drives currently include host-managed SMR disks connected to a
> smartpqi HBA and null_blk block devices with zoned mode enabled.
> Upcoming NVMe Zoned Namespace devices will also be affected.
> 
> Fix this by adding the boolean elevator_init argument to
> blk_mq_init_allocated_queue() to control the execution of
> elevator_init_mq(). Two cases exist:
> 1) elevator_init = false is used for calls to
>    blk_mq_init_allocated_queue() within blk_mq_init_queue(). In this
>    case, a call to elevator_init_mq() is added to __device_add_disk(),
>    resulting in the delayed initialization of the queue elevator
>    after the device driver finished probing the device information. This
>    effectively allows elevator_init_mq() access to more information
>    about the device.
> 2) elevator_init = true preserves the current behavior of initializing
>    the elevator directly from blk_mq_init_allocated_queue(). This case
>    is used for the special request based DM devices where the device
>    gendisk is created before the queue initialization and device
>    information (e.g. queue limits) is already known when the queue
>    initialization is executed.
> 
> Additionally, to make sure that the elevator initialization is never
> done while requests are in-flight (there should be none when the device
> driver calls device_add_disk()), freeze and quiesce the device request
> queue before calling blk_mq_init_sched() in elevator_init_mq().
> 
> Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>

Coincidentally, I had been looking into a problem that is fixed in
5.4-rc1 by this patch.  Thanks for that!

The problem was a delay during boot of a KVM guest with virtio-scsi
devices (or hotplug of such a device to a guest) in recent releases,
especially when virtio-scsi is configured as a module.  The symptoms
look like:

[    0.975315] virtio_blk virtio2: [vda] 1803060 4096-byte logical
blocks (7.39 GB/6.88 GiB)
[    0.977859] scsi host0: Virtio SCSI HBA
[    0.980339] scsi 0:0:0:0: Direct-Access     QEMU     QEMU HARDDISK
2.5+ PQ: 0 ANSI: 5
[    0.981685]  vda:VOL1/  0XA906: vda1
[    0.988253] alg: No test for crc32be (crc32be-vx)
...stall...
[   24.544920] sd 0:0:0:0: Power-on or device reset occurred
[   24.545176] sd 0:0:0:0: Attached scsi generic sg0 type 0
[   24.545292] sd 0:0:0:0: [sda] 385 512-byte logical blocks: (197
kB/193 KiB)
[   24.545368] sd 0:0:0:0: [sda] Write Protect is off
[   24.545416] sd 0:0:0:0: [sda] Mode Sense: 63 00 00 08
[   24.545456] sd 0:0:0:0: [sda] Write cache: enabled, read cache:
enabled, doesn't support DPO or FUA
[   24.547033] sd 0:0:0:0: [sda] Attached SCSI disk

I debugged this down to the same behavior described/fixed back in 3.18
by commit 17497acbdce9 ("blk-mq, percpu_ref: start q->mq_usage_counter
in atomic mode"), and for the same reason.  The delay starts occurring
as soon as q->q_usage_counter is converted to percpu for the one LUN tha
twas found, while scsi_scan_channel() is still working on its loop of
mostly non-existent devices.  Exactly when this problem started
re-occuring is not certain to me, though I did see this problem with 5.2
on linux-stable.

When I run with a 5.3 kernel, the problem is easily reproducible.  So I
bisected between 5.3 and 5.4-rc1, and got here.  Cherry-picking this
patch on top of 5.3 cleans up the boot/hotplug process and removes any
stall.  Any chance this could be cc'd to stable?  Any data someone wants
to see behavioral changes?

Thanks,
Eric

> ---
>  block/blk-mq.c         | 12 +++++++++---
>  block/elevator.c       |  7 +++++++
>  block/genhd.c          |  9 +++++++++
>  drivers/md/dm-rq.c     |  2 +-
>  include/linux/blk-mq.h |  3 ++-
>  5 files changed, 28 insertions(+), 5 deletions(-)
> 
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index ee4caf0c0807..240416057f28 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -2689,7 +2689,11 @@ struct request_queue *blk_mq_init_queue(struct blk_mq_tag_set *set)
>  	if (!uninit_q)
>  		return ERR_PTR(-ENOMEM);
>  
> -	q = blk_mq_init_allocated_queue(set, uninit_q);
> +	/*
> +	 * Initialize the queue without an elevator. device_add_disk() will do
> +	 * the initialization.
> +	 */
> +	q = blk_mq_init_allocated_queue(set, uninit_q, false);
>  	if (IS_ERR(q))
>  		blk_cleanup_queue(uninit_q);
>  
> @@ -2840,7 +2844,8 @@ static unsigned int nr_hw_queues(struct blk_mq_tag_set *set)
>  }
>  
>  struct request_queue *blk_mq_init_allocated_queue(struct blk_mq_tag_set *set,
> -						  struct request_queue *q)
> +						  struct request_queue *q,
> +						  bool elevator_init)
>  {
>  	/* mark the queue as mq asap */
>  	q->mq_ops = set->ops;
> @@ -2902,7 +2907,8 @@ struct request_queue *blk_mq_init_allocated_queue(struct blk_mq_tag_set *set,
>  	blk_mq_add_queue_tag_set(set, q);
>  	blk_mq_map_swqueue(q);
>  
> -	elevator_init_mq(q);
> +	if (elevator_init)
> +		elevator_init_mq(q);
>  
>  	return q;
>  
> diff --git a/block/elevator.c b/block/elevator.c
> index 520d6b224b74..096a670d22d7 100644
> --- a/block/elevator.c
> +++ b/block/elevator.c
> @@ -712,7 +712,14 @@ void elevator_init_mq(struct request_queue *q)
>  	if (!e)
>  		return;
>  
> +	blk_mq_freeze_queue(q);
> +	blk_mq_quiesce_queue(q);
> +
>  	err = blk_mq_init_sched(q, e);
> +
> +	blk_mq_unquiesce_queue(q);
> +	blk_mq_unfreeze_queue(q);
> +
>  	if (err) {
>  		pr_warn("\"%s\" elevator initialization failed, "
>  			"falling back to \"none\"\n", e->elevator_name);
> diff --git a/block/genhd.c b/block/genhd.c
> index 54f1f0d381f4..26b31fcae217 100644
> --- a/block/genhd.c
> +++ b/block/genhd.c
> @@ -695,6 +695,15 @@ static void __device_add_disk(struct device *parent, struct gendisk *disk,
>  	dev_t devt;
>  	int retval;
>  
> +	/*
> +	 * The disk queue should now be all set with enough information about
> +	 * the device for the elevator code to pick an adequate default
> +	 * elevator if one is needed, that is, for devices requesting queue
> +	 * registration.
> +	 */
> +	if (register_queue)
> +		elevator_init_mq(disk->queue);
> +
>  	/* minors == 0 indicates to use ext devt from part0 and should
>  	 * be accompanied with EXT_DEVT flag.  Make sure all
>  	 * parameters make sense.
> diff --git a/drivers/md/dm-rq.c b/drivers/md/dm-rq.c
> index 21d5c1784d0c..3f8577e2c13b 100644
> --- a/drivers/md/dm-rq.c
> +++ b/drivers/md/dm-rq.c
> @@ -563,7 +563,7 @@ int dm_mq_init_request_queue(struct mapped_device *md, struct dm_table *t)
>  	if (err)
>  		goto out_kfree_tag_set;
>  
> -	q = blk_mq_init_allocated_queue(md->tag_set, md->queue);
> +	q = blk_mq_init_allocated_queue(md->tag_set, md->queue, true);
>  	if (IS_ERR(q)) {
>  		err = PTR_ERR(q);
>  		goto out_tag_set;
> diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
> index 62a3bb715899..0bf056de5cc3 100644
> --- a/include/linux/blk-mq.h
> +++ b/include/linux/blk-mq.h
> @@ -248,7 +248,8 @@ enum {
>  
>  struct request_queue *blk_mq_init_queue(struct blk_mq_tag_set *);
>  struct request_queue *blk_mq_init_allocated_queue(struct blk_mq_tag_set *set,
> -						  struct request_queue *q);
> +						  struct request_queue *q,
> +						  bool elevator_init);
>  struct request_queue *blk_mq_init_sq_queue(struct blk_mq_tag_set *set,
>  						const struct blk_mq_ops *ops,
>  						unsigned int queue_depth,
>